It can easily happen. I came within a hair of doing the same thing once, only the guitar was sitting on the trunk area behind the rear window of the car, not on the roof. I drove about 3 miles before I realized it was there, and it never fell off, and that was a miracle.

After that, I had a basic rule: NEVER put the guitar (or anything else you value) on top of or behind the car, ONLY beside the car...if you must put it down at all...and better yet, don't put it down at all. Just don't.

It's a crappy way to lose a good instrument.