Looking for a specific fic in which Dean (possibly Stanford era, spying on Sam) watches Sam pick up a girl and have sex with her in a dark bar where anyone could technically see. I can't remember the author or the title, but I am 99% sure I read it on the AO3 - I've been back through all the relevant tags and can't find it, and it's driving me nuts. It may also have involved either past or heavily implied past wincest, but a key point is that Sam was unaware of Dean's presence.

Any help gratefully received! I'm starting to worry that I hallucinated this damn story!