Undercover police officers that walk around parking lots at concerts or other events looking for any minors drinking alcohol, smoking marijuana, using tobacco, or any other illegal thing you can think of. They will dress in common street attire and typically will be in small groups or spread out in the same area. Will present badge, ask age and for i.d., if you have done anything wrong they will arrest you. If you are a minor, they will usually call your parents and have them pick you up, but if they are assholes they might take you to juvenile detention. If you are an adult, jail. A very ridiculous group of people whom have nothing better to do with their lives than ruin other peoples nights.
Concert Goer #1: Yo man why didn't you meet up with us at the Dave Matthews Band concert?
Concert Goer (myself) #2: Sorry dude I got arrested in the parking lot by the Excise Police because I was smoking a cigarette and I had seven joints in my cigarette pack. I'm in deep shit!
Concert Goer #1: Dude that sucks! That's the second year in a row that's happened to you.