Why do police officers do this?

Why do some police officers touch the back of the car of the person they have stopped as they walk up to ask for license and registration?

to scratch the paint work
[=
mean people
[seriously I don't know]
S=
sorry

Always wondered myself. I think they just do it. Very predictable people.

maybe being in the police you have to be secretly psychic and when they touch the car they can 'see' what you've been doing?!??!? ooohhh!

I don't know, do they? I was pulled over twice and I was so busy flapping about what the people in my car were doing, I didn't notice what the police officer was doing lol!

is it just the states they do this? where is the engine of the car? ooh I don't know!

why would they do that? interesting...can't wait to see a valid reason for this lol!

If I recall it's to see if anyone is in the trunk, if its hot (moving meth labs).

Mainly it's for testing to see if someone has been stuffed in the trunk, you can line a trunk with foam and make it basically soundproof, but if someone is yelling or touching the trunk it will vibrate.

I'v heard that it's to leave finger prints on the vehicle as proof if something should go wrong during the pull over. But like akumo said also testing to see if someones has been stuffed into the trunk or feel the movements of the driver. Maybe as a headsup f the drivers going to try and pull something off. I guess there's a number of reasons... =\

Lol I have a motorbike and have been pulled over on them literally dozens of times, and not yet has a cop EVER touched my bike. If they would, I would grab the knife I have hidden at the right fairings edge and gut em. Well maybe not but you get the idea. And I have a knife on the bike. Nobody touches my bike and lives muahahaha

Been pulled over twice in a car, and I think on the second time, (2 police officers in one car) the second guy went over and did that touch back of the car thing whilst the first asked me questions like "Why you in such a hurry?"

I have never thought of that maybe it's just a dumb habit? Or maybe theres a real reason to it but I wouldn't have a clue as to why.