Trunk tapping is actually a safety procedure! The officer is making sure that no one is hiding in the trunk. Sounds a bit crazy, right? Well, back in the 1970s, street gangs were quite prevalent and patrolled the streets. Members of the gangs would hide in trunks and pop out if a car was pulled over, putting the officer at risk. Tapping on the trunk would surprise whoever was hiding and give the cop a heads up.