It seems (and is logical, I suppose) that you have to match the symbol pretty closely to get what you're looking for. I drew a car twice, the first time getting absolutely nothing relevant, and the second time -- trying to be more precise and using all the space available -- got automobile, taxi, bus, etc, etc.

edit: The primary problem here, I mean, is that if you don't know what the symbol looks like and want to see if it exists, you might not get hits the first time you try to draw, but it might not actually exist anyway.