Logind offers this, through the LockedHint property of sessions: an app can get the ID of the auto session to determine the ID of its session, and then connect to the standard PropertiesChanged signal of that session and look for changes of the LockedHint property.
There’s no portal around this, tho; the app needs an explicit --system-talk-name=org.freedesktop.login1 finish arg in its manifest.