If he's not bothering you, then it's fine. He may be wearing Bluetooth earbuds. Maybe reclining in his car seat is the most comfortable place for him. My own mother jokes about reading in her car on her drive in winter as it gets nice and warm when the sun is out. People do all kinds of things that may seem unconventional to others. It doesn't mean they are a threat. You could try greeting him and just mentioning you noticed him in the car late at night. I bet he gives you a perfectly rational explanation.