When I was in college we changed roommates every semester. I always had to figure out the masturbation habits of my current roommate before I felt comfortable doing my daily recreation. I was pretty shy about being open about it, so unless my roommate was, I had to do it so he couldn't see or hear. That usually meant doing it into a sock at night, very slowly and quietly.
I had a little trouble figuring out this one roommate. I saw no sign of him relieving himself, so got curious. One day when he was out, I decided to look in his laundry bag for clues. One thing that I had noticed was that the first thing he did in the morning was change underwear as soon as he got up - he wore simple white boxers. As soon as he changed, he'd grab his towel and toothbrush and go to the bathroom to shower, etc.
So this day I looked in his laundry bag, and sure enough, the underwear on top had a big wet semen stain in the front. That could have either have been from a wet dream the night before or maybe he masturbated into his underwear and just left them on. I checked the others out, and every pair had a crusty, dry stain in the front. That made it pretty obvious that he was masturbating every night. That turned me on, and I jacked off right into his fresh underwear and put it back.
That night I was determined to see if I could see how he did it so quietly. I pretended to go to sleep, and watched with almost closed eyes. After a little while, I could see slow, gentle motions and a tent down over his crotch. He had put one knee up to make the tent, and I could see his arm just moving a little. He made no sound, but after he did that for about 10 minutes, there was a slight jerking of his bed, a soft moan, and he lay still for several minutes, breathing hard. Then he put down his leg, turned on his side, and went to sleep.
I had my sock at the ready, and was really turned on by watching and hearing him, so I jacked off into it quietly, and went to sleep.
In the morning, my roomate got up and changed his underwear as usual, with the normal morning wood. This time I was watching more carefully, and could see wetness at the opening of his foreskin, which was white and bubbly, and his pubes were matted a little with dried semen. Obviously the wetness in his foreskin was the remains of last nights masturbation, since he didn't wipe up, and the matting was because of the semen he'd ejaculated into his underwear. I was so turned on by seeing that that I jacked off again as soon as he left the room for the bathroom.
That night I watched again, and was rewarded with a similar sight. Though I couldn't see much, knowing what he was doing was a real turnon. I had been thinking about it all day, and anticipating watching him and jacking off myself. I was so turned on I couldn't wait, and started doing it at same time as he was, and we came together. I'm pretty sure he wasn't aware of me seeing him, or what I was doing.
I woke up in the middle of the night, and looked over at him. It was a warm night, and he had pulled back the covers so he was laying, asleep, in just his boxers. I was really curious about the mess in his underwear, so got up very quietly and tiptoed over next to his bed, and reached out slowly. I touched his side, and there was no response, so I gently put my fingers into the slit of his boxers and felt around. It was all wet and sticky in there.
I don't think I could have slept with that wet, cold mess, but it was exciting to feel it. I was able to pull his limp penis out of the slit, and pull his foreskin back a little. Sure enough, it was full of wet semen. I put my finger on his head and rubbed it around a little to feel the wetness, and the softness of his penis head under his foreskin. His penis started to harden, so I pulled my finger out quickly and went back to my bed. I smelled and tasted his semen on my finger, and was very excited. I jacked off again, having an excellent orgasm while smelling his semen and watching his penis, which was semi-hard and still outside his boxers.
I did the same thing on many other nights, and also started to not hide my masturbation as much. He started to see me doing it, and I let him see that I was able to see him, too. So every night we would both do our thing together, not caring that the other saw what was going on. But the one thing he never did know was that I fondled his penis many times while he was asleep. I loved to get it hard, then leave it out to look at while I masturbated, sniffing his fresh semen on my fingers.