I've been walked in on twice and caught once while masturbating.
The first time I had just gotten done with a shower and was standing in front of the bathroom mirror. I was touching my small, A cup breasts for a little while and then let my hand stray down to my pussy. I lost track of time and got caught in the moment, and my brother had opened the bathroom door thinking I was already out. I quickly pressed my towel against my body and yelled, 'Get out!' He didn't catch me masturbating, and even if he did, I'm sure he had no idea what I was doing. Not sure how much of my body he saw, but I didn't worry about it after that.
The second time, I was 16. I was sprawled out completely naked on my bed and rubbing away at my pussy, and playing with my tits, still A cups, from time to time, a good long bit of fun. Then I hear my door open and I jumped to cover myself, both hands over my pussy, ignoring my breasts. It was my mom, and she just said, 'Oh, honey, I didn't mean to walk in like this, I'm sorry.' I didn't care much, it was just my mom, we've seen each other naked before. I just giggled and said, 'Eh, mom, no big deal.' She then came into the room and closed the door. She said, 'I really had no idea you had discovered this at this point.' She sat down next to me and gave me a hug and said that she was proud of me. We had a girl chat moment, and I was not bothered being around my mom naked. I didn't care, it was just my mom. So, sometimes I'll go topless around the house when it's just her at home. And she'll give me warning if she sees anyone else coming home.
The third time, I was 17, and was riding in a car with a bunch of friends, all girls, on a road trip. I was in the back seat of this van we were driving, it was night and very dark. I was starting to feel pretty horny and so I discreetly put my hand into my shorts to rub myself, or so I thought it was discreet. I made it look like I was asleep against the side of the car and curled up. My girl friend next to me totally knew what I was doing, because she jokingly started touching my breast, which grew to be full B's. I looked over at her in panic, and she just motioned that she wouldn't tell. Then she went back to talking with people in the middle seats. She paid attention to me again when she could tell I was getting closer to orgasm, and she just cuddled against me and discreetly felt up my breast and tweaked my hard nipple as I came.