This happened to me when I use to do outcalls at the motel with the 4 hr siesta rate. I remember we were getting things underway and we were both naked and I was lying on the bed and she was kneeling beside me and showing me her oral skills. Then a knock came on the door and she stopped, I didn't do anything thinking they would just go away if no one answered. Next thing I know, the guy from the reception walks in on us. I'm lying on the bed with a hard on, she's kneeling on the bed naked and in shock that the guy walk in on us. There was a pause that lasted an eternity and then he turned away and closed the door behind him. Then the girl starts freaking out "OMG, he saw my face....". I was like WTF he doesn't know who you are and I get her to continue the BJ. After a minute or so we forgot about the incident and continued on our main event.
Moral of the story, when getting a room always use the security lock or chain just in case reception or house keeping decide to pay a visit. I think the reception guy was more embarrassed than me when I had to go return the key and remote.