It's been a few years since I've done that, but normally gauged by the shouting and speed of thumps emanating from behind the wall.
I've had a friend walk in while I've been 'on the nest' as it were, looking for skins. He's trying to hold a conversation with me while I am otherwise engaged in thinking other far more furtive thoughts.
It wasn't until I stopped looked up at him and told him to f**k off that he noticed (how he didn't at first I don't know). He stopped and stared unsure what to do or say. My then girlfriend, who was a complete star btw, pushed me back lifting the sheet at the time to show her (rather nice, I have to say) breasts and said, "there you go, if you want something to look at, now f**k off!"
Needless to say his embarrassment broke the spell and he hurriedly shuffled away. I did hear him stopping others coming into our room a couple of hours later too ..