Consequences for 12 year old with ASC

Also, to clarify, by way of making it clear earlier that he is struggling, that isn't necessarily "you needing to notice earlier"- it's just as likely it may mean working on improving his self-reflective strategies so that HE knows earlier when he is struggling.
It could be that he rejected the strategies because you spotted the difficulty and he didn't. We do often have trouble identifying our own emotions and stress levels, especially as youngsters. :/ 

That does make things harder, especially as your other boy is autistic too and I imagine having to stop what he was doing to follow his sibling with you would cause considerable distress.

Is there someone you could leave the 12 y/o with (a grandparent?) next time you have an outing that would be with both boys and only one parent? Just that one time, then he could come with you again the next time. That would bring home the message, I think, without being too confrontational.

If he's rejecting the strategies you have, he probably needs new ones for some reason (or a way to make it clear that he is starting to struggle earlier in the process, as I would often as a child reject any idea when imminently close to meltdown due to sheer lack of capacity to process it with my already overloaded brain).
When he's calm, talk about why he rejected those strategies and come up with some new ideas together.

And just tell him in stark terms what the consequences could have been if the worst HAD happened. 

Thanks for your responses. I wanted to clarify that I was on my own with the boys. My husband was not there. I couldn't just leave unfortunately. I could see he was having problems before hand and I offered him a few of our normal tactics to help him but he rejected them. I also understand that he can't control himself when he's having a meltdown and wouldn't ever give him consequences for a meltdown. However I also know (because I know him well and have talked to him) that he did make a decision to disappear and I want him to understand why that is not acceptable. Especially when he took my bag with him with my purse and keys in. 

I agree with Dragoncat, the thing about meltdowns is that you really have zero control over what you're doing as you experience them. It's like you're just observing your body being controlled by a terrifyingly reckless and aggressive autopilot; it's actually quite an unnerving experience. Then you get control back but your batteries are completely drained and you are very aware that you are about to deal with the consequences of what your body did on its own.

I definitely understand that what your son did was very dangerous and must have scared you both dreadfully. 
I do think, however, that the most productive approach is to work out what his triggers are (is it when it's crowded, noisy? Is it when the schedule- or what he thinks is the schedule- suddenly changes?) and work out a strategy together to avoid them as much as possible and have an emergency plan in place.
Consequences won't help- he will know full well he's done something he shouldn't, albeit when he wasn't in control.

Perhaps if/when he walks off again, now you know he has the potential to go far, would be for one of you to just follow at a distance (and make him aware beforehand that you won't try to interact with him, just be close enough to keep him in sight).

If he had a meltdown, it wasn't his fault, so imposing consequences could be unhealthy and damaging for him. Instead of punishment, perhaps together you could find a way to deal with such occurences in the future, possibly some kind of signal he can give you when he's starting to feel overwhelmed, so that you can all find a safe place until he has calmed down.