Similar as Gramps above - way back when, my GF's parent's house had a carpeted and panelled basement with a couch, a couple of chairs and a TV and that is where my GF and I spent our time working on the night moves most every weekend. After watching TV for a bit in the evening [typically after coming back from a party or bar] we would turn out the lights before getting amorous, using the light from the TV for illumination. One evening when we thought all had either gone to bed or had succumbed to excessive alcohol [both her parents drank] we were well along in a horizontal dance on the couch when we hear a gasp from the foot of the stairs. We both froze in shock, looked up trying to make out in the darkness who was there [the TV screen had gone fairly dim as it must have been a night scene in the movie that was on] fearing the worst that it was her mother. Turned out to be my Gf's younger sister who had very quietly come down the basement stairs and turned the corner into the basement room. The stairs were completely enclosed in panelling so she could not see into the basement until she turned the corner at the bottom. My GF and I both yelled at the sister to get upstairs and not to say anything. Never heard if anything was spoken of the incident beyond that and everything seemed normal with the younger sister for the rest of the time my GF and I were together.
As an aside, one time a couple of years later when in that basement I noticed the couch slip cover had stains all over it. I was truely shocked I had not noticed it before or nobody had ever said anything. Realized at that moment both her parents had to know what we had been doing in the basement all that time.
