I think I would have been too shocked to have thought about ringing the girl's parents. Though I can see that would have been nice if they'd been up all night and if the girl was prepared to give me their phone number. But I don't know any 16 (or 14) yr olds that don't have their own phones they could have called home on if that were something they wanted to happen.
The OP saw a situation which she read as a teenager having a few too many and sleeping it off in the OPs car without permission. It may have been a different scenario, but given the OP was the only one there, and hasn't mentioned any details that contradict her reading, everybody else's second guessing is unreasonable.
As others have noted "letting" a 16 (or 14) yr old walk home, even a long way, on a sunny day is not dangerous or unreasonable. And I don't buy this "If you think she's 16 she's probably 14" reasoning. I routinely think people are a lot younger than they are. If I think someone's 16 she's almost certainly at least 18, probably 20+.
It's always possible the OP missed a chance to provide support after a terrible incident. But it's not particularly likely and given the OP was caught totally by surprise and did not see any signs that indicated something had happened, it's silly to berate her for it. On the face of it the girl had a rollicking good night and a safe, if chilly and uncomfortable, kip somewhere she ought not to have. It isn't a terrible scenario. The sort of worst first thinking some posters engage in, insisting it is "bad" of the OP not to have been actively enquiring after the girl's welfare, is stifling and fails to let people take joy in the pleasures of youth and exploring the world.
But YABU OP. It's a funny story. There was no damage and it will give you plenty of laughs and a good tale to tell. Why be pissed off about that :)