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You could also detect when the elevator call button gets pressed again right after the elevator has left a floor. That means someone wasn't able to get on the elevator (and is now waiting for the next one).

> If it was a camera, security issue

What security issue? Many hotel elevators already have cameras.

Because there would need to be some automated service and you best believe it'll be flock 2.0

This could easily be done locally with no network

I mean this in the nicest way possible. Do you see all these cloud photo/video leaks like from robot vacuums or whatever and think they don't know it can be done locally?

Following rz2k's suggestion, you could have a building employee operate the elevator during busy times.