How would you react if you caught someone 'in the act'?

So earlier today I was walking down a stairwell to get to my car which was parked at the campus underground garage. When I turned to climb down the last set of steps I heard shuffling and instantly got nervous because I’m always creeped out by this stairwell. So then this teenage guy pops out from behind the corner and stares at me blankly, and behind him is a naked teenage girl trying to cover herself. I just ignored them and kept walking to my car because I had no idea how to react.
This is a garage I come out of several times each day,I’ve seen condom wrappers on the ground in the stairwell before but never thought much of it. There’s no attendant (it’s all electronic) to report to, so I’m thinking I’ll start using the elevator…?
What would you have said or done if you were the one to interrupt?