Also, as long as you don't have a warrant and the piece is out of sight (in the glove compartment, backpack, etc.) they can't search your car without your permission. They can ask you out of your car, but close and lock the door if you do, and don't give them permission to search your car, say no.
also, if the pipe is completely clean of res, then you're straight, no matter if it's in plain sight.