Legality of hidden microphone in the bathroom?

Without going into too much detail, my co-workers and I have reason to believe our employer put a microphone in the employee bathroom/locker room, presumably to monitor us because they never go in there and/or to catch us breaking rules, etc. I know it's illegal to have a camera in a bathroom, but could someone tell me if eavesdropping on employees in the bathroom in a concealed fashion is legal?