you mean the NMPD beacon? that one is a sniper rifle dangling from a power line. jump on the truck right in front of it and you will be able to hit
to start it
also to the person asking about multiplayer achievements... yes you can. my buddy confirmed it with me that he unlocked a couple today with that disc. I will add both these questions to the FAQ
I am going to add what the beacon/item thingy looks like for each mission but I cannot do that till I get home since I am at work now (it is a slow day).