Red door alarms in the shape of a stop sign.

Have you seen them? A local store has one on the emergency exit door.

Can you post a picture?

Is it this one?
https://www.sti-usa.com/series/sti-exit-stopper/

Yes that is it.

Have to say I have never seen that sort of door alarm before.

True. I’ve only seen the red/orange light sounders in stores like the Home Depot and everywhere else those emergency alarms that look like EST Genesis speaker/strobes.

Here’s a video about it.
https://www.youtube.com/watch?v=feP_v4Rw-sE