What could possibly be causing the burn marks on the ceiling of my car?

The only thing I can think of is that one of your friends is messing with you. If the burn mark is not sooty on the underside but it is on the side you can see, it has to be caused by an outside heat source. Maybe if you get out of the car, and they stay inside it, they take the lighter and move it back and forth creating a long burn mark. Not enough to cause a fire. Just enough to make a mark. It is either that or they have some kind of chemical that would do it. But I don't know what would be able to do it that is readily available.