This happened just a year or so ago.
My son had just reached his twenties when he started dating Amy, who was in her last year as a teenager. For several months they would see each other a couple of times a week and go out somewhere, but as the colder weather began to come around they would spend the evening in his bedroom. Amy was a petite girl with short dark hair. She could easily pass for someone several years younger.
It was a Saturday morning, my son was at football practice and my wife had gone into town for the day, which pretty much meant I had several hours to myself. I was about to check my emails on the family PC when I noticed a USB memory stick in the port on the front of the computer tower. Figuring it belonged to my wife as she often worked from home, I clicked to open it just to make sure it was hers, and found three movies files. This didn't look like something she would have so I clicked to start the first one.
It opened up with a girl with her back to the camera, leaning on a chair, dressed in what could only be described as a school outfit. White shirt, dark blue short pleated skirt, white knee socks and black pumps. Some background music began to play and she began gyrating around, still with her back to the camera. Occasionally she would bend over and show her little white panties. This continued for couple of minutes and I could feel my cock thickening, until she turned to face the camera. It was Amy, my son's girlfriend!
It was then I realised the background was my son's bedroom. Amy continued gyrating around like some stripper in a bar, feeling her little breasts through her shirt and running her finger up and down her panty-clad pussy. She slowly undid several buttons on her shirt revealing her little B-cup titties. Her nipples were rock hard as she teased and pinched them. My cock was rock hard by now, and with a quick glance around I dropped my pants and freed it of it's confines. I started stroking my cock, not realising my luck that my son's teenage girlfriend was slowly stripping off. Her skirt came off leaving her in shirt, panties and socks, quickly followed by her shirt. Her lithe little body got closer to the camera whereupon she turned her back to it and slowly peeled her panties off. Her little pussy was framed beautifully between the top of her legs and my cock was now dribbling pre-cum.
She slowly turned to face the camera and stepped back to sit in the chair. She opened her legs to reveal her bald, little pussy. She began to ride her finger up and down her wet slit, occasionally pushing the finger inside her. Her frigging became more frantic as she played to the camera. My hand was now a blur as my cock was ready to burst. Amy was now plunging two fingers into her pussy as my cock released several ropes of cum across my groin and lower stomach. She moaned loudly as I was cumming, almost in unison with me.
The movie file stopped there, as I began to clean myself up. I quickly grabbed a spare USB memory of my own, copied the files for my own use, and shutdown the PC. When my son returned at lunchtime, he went straight into the study before going to his room to change. It was then I noticed the USB stick had been removed! I still had another couple of movie files to watch though!