I don't know how common it is, but I have a huge fetish about telling people when I have masturbated. I guess it is a sort of after the event exhibitionism!
It all sort of started when I was a kid aged about 13. I got caught playing with myself by my mother. She said nothing about at the time, but later in the day she had a chat with me about it. It was very embarrassing at the time, but she was actually quite enlightened: she basically told me it was no big deal but not something to do in public. The weird thing was I got a huge erection during the chat and had to go and cum again when she had left. Over the next few months I let her "catch me" several times just for an excuse to tell her "sorry" later. It was not getting caught that was the turn on, so much as the red faced "sorry Mum I didn't mean on you to catch me doing that". I once was really daring and apologised to her when one of her friends was in the next room and I knew she could hear me.
Now days I am married and have shared this fetish with my wife and she indulges me in it. It is now almost part of our bedtime ritual for me to tell when, where and how I have done it during the day, and she does the same for me.
I have just realised that in typing this I am acting out that same need. Guess what I am going to do again now?