Seems like I should know what this is. I saw two of these in the house I inspected Saturday. It is the size of a switch or outlet cover, smooth white with yellow, green and red lights (not lit up when I was there). Sorry for the poor picture quality…
Maybe a CO detector? looks similar to one I saw that was part of a alarm system.
Glass break detector.
What Roy said. 
Good one Roy!
I use to own a security alarm company what is why I recognized it.