I've noticed something similar. I sometimes take a nap in my car for my lunch break at work, and I have had some strange dreams. What makes them a little more freaky is that they tend to include me driving my car somewhere so I think that I'm really driving my car while I'm asleep so it sometimes can be a little scary and disorienting. Usually though my car doesn't seem to be reacting properly to everything I do, so I guess that would make a pretty good dream sign.