Do not set your half-eaten burrito and a can of Sprite on the trunk of a parked car at two A.M..
In general you shouldn’t sit on stuff or put things on top of anything that you don’t own. It’s rude, assuming, and selfish. If it’s a really nice car or it has a stupid vanity license plate like “SMLPEEN” owned by someone who probably beats off to Barstool Sports videos, I understand the temptation. But I also guarantee that this person will call their mom to kick your ass.