Peep holes.
 

I went to my local supermarket the other day (in the UK), popped to the loo and noticed a hole slightly bigger than a screw hole that looked from the end cubical out across the two urinals. I think you could get a pretty good view of who was using them if you wanted. Are these common in bathrooms?

I am really interested in finding a gloryhole which women use, however not found any in the UK, only this peep hole which looked out over the wrong kind of plumbing for my liking lol.