Yeah, that's a good idea once motion has been detected by a computer.

I just saw a Nova program about elevators where this guy was stuck in an elevator for 41 hours and there were paid humans on duty the entire time who had video they never looked at.

Here's the story:

  http://well.blogs.nytimes.com/2008/05/21/trapped-in-an-elevator-captured-on-video/

I think the case was settled for $200,000.

A motion detector would have more reliably sounded the alarm or some electronic sensor indicating the elevator was stuck. I have to wonder sometimes if people in old industries like this know what's going on.