Sounds like some of the guys who troll for "dates" at a park, here in Dallas. It's pretty widely known that if you see a guy at this park, after hours, sitting in a car that is backed into a parking spot, he's hoping for an encounter.

There's even a local radio station based in the building accross the street and they used to do a bit called the "Hey Man, What's Up Rodeo". They'd put a wire on a guy and he'd head over to the park, approach a vehicle and say "Hey man, what's up?" Then he'd try to keep a conversation with the guy for as long as possible, take it as far as he could without actually having sex with the guy. Sometimes it was funny, sometimes boring as all get out.

He's probably living a "straight" life, goes out to try and pick up guys that he won't ever have to see again or encounter in his day-to-day activities.
Creepy? Yes.
Illegal? No.
However, if the cops talk with him, he'll likely find a new place to wait for his fun.