Oh, that sounds like a horrible experience and trigger!

Are you sure you cannot open the doors from the inside when they're locked? Most cars allow the inside openers to work, maybe not in the back "child" seats but most front seat passenger side doors can open or the knob pulls up, etc. I think it's probably against the law for them not to open (unless it's a police car :-)

What I would do is make sure I had a key to that car; I always have my key to our car as DH is too slow to let me in when we're somewhere or, like you, he'll go off and I'll want to start the engine so I can put the windows down, etc. Usually though, I just open the door a bit when H gets out to get gas or whatever and I'm stuck in a "dead" car.