If that was shot in one of the rooms in the basement of the apartment building you live in, you might have taken a bit of a risk. What if somebody had barged in in the middle of the shoot? Well, maybe that was the basement of your parents' house or some other private space. What I'm saying is that it's always a good idea to keep in mind the legality and logistics of your photography.
Your first guess was right. I was being really quick with my shot because of course I wouldn't want anyone walking in on me like that. There is definitely nothing illegal about taking a picture in your apartment building.
"There is definitely nothing illegal about taking a picture in your apartment building."
The risk is small, of course, but imagine some flower hat walking in on you in the basement with you not wearing a single thing.