All the time actually.

I make a point of watching out for laundry baskets and strewn clothes on the floors of my straight mates' bedrooms.  A couple of times, Ive even taken the found underwear home.

A couple years back, I found a pair with dried cum on them. Clearly he had jacked off and slipped his still leaking dick back into his underwear. I couldnt resis and took them home.

I spent hours with those underwear. The friend who owned them confronted me about a missing pair of underwear so I admitted to taking them and even went into my bedroom to return them. I took a huge whiff of the crotch and told him I couldnt help myself. That i had always wondered what his nuts smelled and tasted like. He was a little taken aback, but he kept asking me questions about what I liked about his and I kept filling him with complimentary remarks telling him he smelled clean and musky and the cum still tasted great.

When he left, he forgot the underwear on the arm of the sofa he had been sitting on. I  grabbed em up and took them into my bedroom and blew another huge load thinking about him knowing what I was doing.