diff --git a/lib/check.ml b/lib/check.ml
index c51d5690..01f3e36c 100644
--- a/lib/check.ml
+++ b/lib/check.ml
@@ -3585,17 +3585,33 @@ let refuse_frame_escapes (f : Tast.fn) =
      most one binding [Let] in the function. *)
   let binds = Hashtbl.create 16 in
   let unstable = Hashtbl.create 16 in
+  (* A place a global owns, and so one that outlives every frame: the global,
+     a field or array element of it, or an element of a Vec it holds — the
+     Vec's block is the global's for as long as the global keeps it. A slice
+     is not stepped through: its storage may be anyone's. *)
   let rec place_global = function
     | Tast.Pglobal g -> Some g
     | Tast.Pfield (t, _) -> expr_global t
-    | Tast.Pindex (t, idx) when all_array t.Tast.ty idx -> expr_global t
+    | Tast.Pindex (t, idx) when owned_levels t.Tast.ty idx -> expr_global t
+    (* [(set (at v i) x)] on a Vec is a store through the checked element
+       address [vec_at] builds. Any other pointer's target is not known to
+       be the global's. *)
+    | Tast.Pderef { Tast.e = Tast.Prim (Tast.Rt "flan_vec_at", t :: _); _ } ->
+      expr_global t
     | _ -> None
   and expr_global (e : Tast.expr) =
     match e.Tast.e with
     | Tast.Global g -> Some g
     | Tast.Field (t, _) -> expr_global t
-    | Tast.Prim (Tast.At, t :: idx) when all_array t.Tast.ty idx -> expr_global t
+    | Tast.Prim (Tast.At, t :: idx) when owned_levels t.Tast.ty idx ->
+      expr_global t
     | _ -> None
+  and owned_levels ty = function
+    | [] -> true
+    | _ :: rest ->
+      (match ty with
+       | Types.Array (_, el) | Types.Vec el -> owned_levels el rest
+       | _ -> false)
   (* [(at a i j)] is one node carrying every index; each level stepped must
      be an array for the element to be inside [a]'s own bytes. *)
   and all_array ty = function
@@ -3765,24 +3781,46 @@ let refuse_frame_escapes (f : Tast.fn) =
                  ^ ", instead of its address: drop the addr"))
         (escapes 0 e)
   in
+  (* Only a store into the global itself can be fixed by changing the
+     global's type; for a field, an element or a push, the value's type
+     belongs to something else. *)
+  let stored ~bare g v =
+    Option.iter
+      (fail ~verb:"stores" ~target:(" into the global " ^ g)
+         ~fix_slice:(fun c ->
+             "Store a copy that outlives the frame: " ^ c
+             ^ ", which puts the elements in the context allocator")
+         ~fix_addr:(fun t ->
+             if bare then
+               "Store the value instead: declare " ^ g ^ " as " ^ t
+               ^ " and drop the addr"
+             else "Store the value instead of its address"))
+      (escapes 0 v)
+  in
   let returns = not (Types.equal f.Tast.ret Types.Unit) in
   List.iter
     (Tast.walk (fun (e : Tast.expr) ->
          match e.Tast.e with
          | Tast.Return (Some v) when returns -> tails v
          | Tast.Set (p, v) when not (String.equal f.Tast.name "main") ->
-           (match place_global p with
-            | Some g ->
-              Option.iter
-                (fail ~verb:"stores" ~target:(" into the global " ^ g)
-                   ~fix_slice:(fun c ->
-                       "Store a copy that outlives the frame: " ^ c
-                       ^ ", which puts the elements in the context allocator")
-                   ~fix_addr:(fun t ->
-                       "Store the value instead: declare " ^ g ^ " as " ^ t
-                       ^ " and drop the addr"))
-                (escapes 0 v)
-            | _ -> ())
+           Option.iter
+             (fun g ->
+                stored ~bare:(match p with Tast.Pglobal _ -> true | _ -> false)
+                  g v)
+             (place_global p)
+         (* A push or put into a container a global owns: the element was
+            bound to a slot first, and its address is what the call takes. *)
+         | Tast.Prim (Tast.Rt ("flan_vec_push" | "flan_map_put"), target :: rest)
+           when not (String.equal f.Tast.name "main") ->
+           Option.iter
+             (fun g ->
+                List.iter
+                  (fun (a : Tast.expr) ->
+                     match a.Tast.e with
+                     | Tast.Prim (Tast.AddrOf, [ v ]) -> stored ~bare:false g v
+                     | _ -> ())
+                  rest)
+             (expr_global target)
          | _ -> ()))
     f.Tast.body;
   if returns then
diff --git a/test/test_flan.ml b/test/test_flan.ml
index 4135988e..5b63acd7 100644
--- a/test/test_flan.ml
+++ b/test/test_flan.ml
@@ -3405,6 +3405,29 @@ let () =
     "(defonce g [i32]) \
      (defn f [] () (let [a [1 2 3] b [4 5]] (set g (slice a)) (set g (slice b))))"
     ~needle:"f stores a slice of a into the global g";
+  rejects_check "a local's address into a global array's element"
+    "(defonce gs [2 (Ptr i32)]) \
+     (defn f [] () (let [x (i32 1)] (set (at gs 0) (addr x))))"
+    ~needle:"Store the value instead of its address";
+  rejects_check "a local's address into a global struct's field"
+    "(defstruct Q [p (Ptr i32)]) (defonce gq Q) \
+     (defn f [] () (let [x (i32 1)] (set (.p gq) (addr x))))"
+    ~needle:"f stores the address of x into the global gq";
+  rejects_check "a local's address into a global Vec's element"
+    "(defonce gv (Vec (Ptr i32))) \
+     (defn f [] () (let [x (i32 1)] (set (at gv 0) (addr x))))"
+    ~needle:"f stores the address of x into the global gv";
+  rejects_check "a local's address pushed onto a global Vec"
+    "(defonce gv (Vec (Ptr i32))) \
+     (defn f [] () (let [x (i32 1)] (push gv (addr x))))"
+    ~needle:"f stores the address of x into the global gv";
+  rejects_check "a local's address put into a global Map"
+    "(defonce gm (Map i32 (Ptr i32))) \
+     (defn f [] () (let [x (i32 1)] (put gm 1 (addr x))))"
+    ~needle:"f stores the address of x into the global gm";
+  accepts "a pointer parameter pushed onto a global Vec"
+    "(defonce gv (Vec (Ptr i32))) \
+     (defn f [p (Ptr i32)] () (push gv p))";
   rejects_check "the fix keeps the slice's bounds"
     "(defn mk [a [4 i32]] [i32] (slice a 1 3))"
     ~needle:"(clone (slice a 1 3))";
diff --git a/test/test_sanitize.ml b/test/test_sanitize.ml
index 529864d9..5e5b61bf 100644
--- a/test/test_sanitize.ml
+++ b/test/test_sanitize.ml
@@ -64,7 +64,7 @@ let run exe args =
   (try Sys.remove out with Sys_error _ -> ());
   (code, text)
 
-let compile ?(dev = false) ~sanitize ~checks path =
+let compile ?(dev = false) ?(opt = "-O0") ~sanitize ~checks path =
   let exe =
     Filename.concat scratch
       (Printf.sprintf "flan-san-%s%s-%s"
@@ -73,10 +73,15 @@ let compile ?(dev = false) ~sanitize ~checks path =
          (Filename.remove_extension (Filename.basename path)))
   in
   let p, csrcs, lflags = Test_support.linked ~dev path in
+  (* -O0 by default, both builds: detect_stack_use_after_return (see [env]) can only
+     report a read of a frame's variable that still has a stack slot. At -O2
+     the local is promoted to a register and the read folded to the value it
+     held, so a pointer into a returned frame reads nothing the fake stack
+     can poison. *)
   ignore
     (Build.executable
-       ~opts:{ Build.default with checks; sanitize; dev } ~csrcs ~lflags p
-       ~out:exe);
+       ~opts:{ Build.default with checks; sanitize; dev; opt }
+       ~csrcs ~lflags p ~out:exe);
   exe
 
 let contains = Test_support.contains
@@ -107,6 +112,9 @@ let reported text = List.exists (contains text) markers
    calc-me is here and is not in test/programs: it is the one string parser in
    the corpus, which makes it the likeliest to push [scratch] or [escaped]
    anywhere near their bounds. *)
+(* temp-grow and condition-temp read text after a (free-temp) on purpose, to
+   show the dev poison, and a sanitized build reports exactly that read: they
+   stay out of this list. *)
 let corpus =
   [ (* bounds.flan selects its case from argv, and every case but 0 is one
        that traps. Argument 0 is the in-bounds path — the last index of a
@@ -327,7 +335,7 @@ let dyn_sweep () =
   let p, csrcs, lflags = Test_support.linked path in
   match
     Build.executable
-      ~opts:{ Build.default with Build.sanitize = true }
+      ~opts:{ Build.default with Build.sanitize = true; opt = "-O0" }
       ~csrcs:(csrcs @ [ "dyn_ops.c" ]) ~lflags p ~out:exe
   with
   | exception Failure m -> fail "dyn: sanitized build: %s" m
@@ -439,8 +447,8 @@ let dev_sweep () =
    sanitized run must produce ASan's report and must NOT produce the
    handler's line, and that is the assertion.
 
-   And it has to be built at -O0. At the sweep's -O2 a store through a null
-   pointer is undefined and need not fault — so a case that is about what happens
+   And it has to be built at -O0, as the sweep now is. At -O2 a store through
+   a null pointer is undefined and need not fault — so a case that is about what happens
    on a fault has to be compiled where the fault happens. Same family as the
    -O0/-O2 split [unchecked_controls] records for bounds.flan.
 
@@ -592,7 +600,8 @@ let control ~expect_report ?(args = []) ~why name src =
    wrong answer instead of touching a redzone. At -O0, with the load still in
    the program, ASan reports it. That divergence is why [--sanitize] does not
    force -O0 the way [--debug] does — the optimiser is half of what is being
-   measured. *)
+   measured — and why this one control is built at -O2 while the sweep is
+   not. *)
 let unchecked_controls () =
   let reports = [ "3"; "7"; "4" ] in
   let silent =
@@ -616,7 +625,7 @@ let unchecked_controls () =
              here and now for a better reason. The trap itself is asserted in \
              test_acceptance.ml" ]
   in
-  match compile ~sanitize:true ~checks:false "programs/bounds.flan" with
+  match compile ~opt:"-O2" ~sanitize:true ~checks:false "programs/bounds.flan" with
   | exception Failure m -> fail "unchecked bounds.flan: build: %s" m
   | exe ->
     List.iter