I just met a guy who is a Cambridge graduate and who doesn't know I know he is one. I know because other people I work for told me.
Prior to our meeting I was wondering how long it would take him to innocently mention this fact and to be honest even I was surprised at how quickly he went for it, through an open-ended observation about the effect of University on his accent within the first 2 minutes. Unsure I'd be able to control my irritation I rapidly turned the conversation elsewhere then buggered off.
Still, the all-time-greatest gratuitous reference to someone's Oxbridge background was at an unbearably tedious Guardian journalist party I went to several years ago.
It literally went like this.
Bloke: Hi. How do you know Paul? (the host)
Me: My wife works with John (his boyfriend)
Bloke: We were at Cambridge together.