diff --git a/scripts/check_buildbot_system.py b/scripts/check_buildbot_system.py
index 3346f98a..4958def2 100644
--- a/scripts/check_buildbot_system.py
+++ b/scripts/check_buildbot_system.py
@@ -205,10 +205,17 @@ def main() -> None:
             sys.exit(1)
 
     # Unreachable upstream means the freshness question was not answered, and
-    # a zero exit says it was answered "fresh".
-    if report.get("error"):
+    # a zero exit says it was answered "fresh". A new asset nobody tracks, or
+    # a tracked one the buildbot dropped, is a change to act on: a zero exit
+    # there let the online pipeline call the system directory current.
+    if report.get("error") or pending(report):
         raise SystemExit(1)
 
 
+def pending(report: dict) -> list[dict]:
+    """Entries a maintainer has to act on: not tracked yet, or gone."""
+    return [e for e in report.get("entries", []) if e["status"] in ("NEW", "STALE")]
+
+
 if __name__ == "__main__":
     main()
diff --git a/scripts/check_freshness.py b/scripts/check_freshness.py
index 5ba10aeb..801a0c69 100644
--- a/scripts/check_freshness.py
+++ b/scripts/check_freshness.py
@@ -685,9 +685,33 @@ def check_data(offline: bool) -> list[Finding]:
             findings.append(Finding("data", key, STALE, detail="upstream changed, refresh_data_dirs.py fetches it"))
         else:
             findings.append(Finding("data", key, OK))
+    findings.append(_check_buildbot_system())
     return findings
 
 
+def _check_buildbot_system() -> Finding:
+    """The buildbot's system directory against what _data_dirs.yml tracks.
+
+    The registry entries are refreshed above; an asset published since, or
+    one the buildbot no longer serves, is only seen by reading its .index.
+    """
+    import check_buildbot_system  # noqa: PLC0415
+
+    with contextlib.redirect_stdout(io.StringIO()):
+        report = check_buildbot_system.check(
+            str(REPO_ROOT / "platforms" / "_data_dirs.yml")
+        )
+    if report.get("error"):
+        return Finding("data", "buildbot system", UNKNOWN, detail="index unreachable")
+    todo = check_buildbot_system.pending(report)
+    if not todo:
+        return Finding("data", "buildbot system", OK)
+    names = ", ".join(f"{e['status'].lower()} {e['zip']}" for e in todo[:SHOWN_NAMES])
+    more = ", ..." if len(todo) > SHOWN_NAMES else ""
+    return Finding("data", "buildbot system", STALE, local=f"{len(todo)} to track",
+                   detail=f"{names}{more} (_data_dirs.yml)")
+
+
 # --- catalogs and recipes ---------------------------------------------------
 
 
diff --git a/tests/test_check_freshness.py b/tests/test_check_freshness.py
index 4117a9ea..97add807 100644
--- a/tests/test_check_freshness.py
+++ b/tests/test_check_freshness.py
@@ -381,5 +381,28 @@ class AStaticTargetStillNamesItsCores(unittest.TestCase):
         self.assertLess(static, source.index("_declared_cores(platform)"))
 
 
+
+class TheBuildbotIndexIsRead(unittest.TestCase):
+    """A new asset in buildbot's assets/system was seen only by
+    check_buildbot_system, which exited 0, as a non-blocking step."""
+
+    def test_new_and_dropped_assets_are_stale(self):
+        from unittest import mock
+
+        import check_buildbot_system as cbs
+
+        report = {"entries": [
+            {"zip": "New.zip", "status": "NEW", "key": None},
+            {"zip": "Old.zip", "status": "OK", "key": "old"},
+        ]}
+        with mock.patch.object(cbs, "check", lambda registry: report):
+            finding = cf._check_buildbot_system()
+        self.assertEqual(finding.status, cf.STALE)
+        self.assertIn("new New.zip", finding.detail)
+        with mock.patch.object(cbs, "check", lambda registry: {"error": "x", "entries": []}):
+            self.assertEqual(cf._check_buildbot_system().status, cf.UNKNOWN)
+        self.assertEqual([e["zip"] for e in cbs.pending(report)], ["New.zip"])
+
+
 if __name__ == "__main__":
     unittest.main()