Your son didn't do anything terrible, he just didn't know what to do when you weren't there and, being a teenager, decided to take matters into his own hands. He made a decision and inconvenienced you. The worry part is your own choice. Sorry, but you are going to have to get used to it.
I'm on my 4th teenager. They all do these things because they are learning how to make decisions, be independent (not a bad thing), and they don't have alot of common sense yet. As a mom, I've found it's best to not think the worst has happened. Usually, there is a logical explanation for them being gone, late, not where you thought you were supposed to meet them, not answering their cell phone.... etc. You can try to plan ahead for every possibility and you will still be surprised that things get messed up.
Try to see this from your son's perspective. He was not out vandalizing or taking drugs. He just didn't know what to do and made the best decision he could at the time. And he might have guessed right, because for all he knew, something bad could have happened to make you late and he'd be standing outside by himself waiting for hours. I know it's upsetting from your perspective and I've been there and done that. But this is a chance to troubleshoot your communication, maybe get him a cell phone, and let your son make some innocent mistakes as he figures out how to manage his world.