This mission is given by Eddie Scarpa.
"Joe, some fuck just hit one of our armored trucks before we got a chance to unload the guns. Had to be an inside job. I don't need this bullshit right now. I don't care what you do or who you gotta kill, but you get our guns back, now!"
Begin the mission and drive around the corner. There's two entrances to the alley where the armored truck is parked and the one to the east is only guarded by one thug. Park back a bit and sneak up on him for a stealth kill. Once he's gone take cover by the wall and the rest will run straight to you. Keep an eye on your mini-map though, occasionally one may run around and come up behind you.
Once you've dealt with them all, get in the truck and drive to the Falcone Garage to end the mission.