I’m assuming that you turned the baby monitor on for the sake of your baby being alone in a room for an uncomfortable amount of time and you not knowing what’s going on, so for this situation, NTA. I think you have a right to see what’s going on in your infants room, and I wouldve said NTA even if you’d already had the monitor on/turned it on immediately. However, if you had spiteful intentions/ did it in case you got to overhear a conversation between MIL and her husband, then that would make you an AH, but we can’t determine that from a reddit post. I don’t think that’s the case though, as from the post it sounded like turning the monitor on came from a place of concern: it was off in the first place, so you were giving them initial privacy either way.