What triggers the message "your screen is being observed" on the login screen

I noticed that macOS (Big Sur) is often telling me on my login screen that "your screen is being observed".

I'm assuming that this is due to the fact that I allowed some apps top record my screen, but that explanation doesn't quite make sense because

1. the message does not always appear (but the permissions are obviously always on)
2. the message states that my screen "is being observed", implying that this is happening right now. But none of the apps that have permission is actually recording.

So if permitting at least one app to record the screen is a necessary condition for the message to appear, it is definitely not a sufficient condition.

My question: what (other) condition(s) must me met for the message to appear?

Bonus question: How can you check which app is "observing" the screen?

Solution 1:

I was able to confirm that OBS Studio does trigger the message even when it is not recording anything. I assume that this is occurs when there is at least one Display Capture Device configured in OBS so that OBS is indeed constantly "observing" the screen.

I did not check whether yet other conditions must be met (e.g. that the source must somehow be active in the current scene). Neither did I check whether the same is true for a Window Capture Device or perhaps even a Browser source, but I guess in many cases pointing to OBS will be enough of an explanation.

I would assume that other apps might trigger the message in similar ways as OBS but I'll leave it to others to identify those apps in separate answers.