Was sitting in the gate pretty much across from Customs, just to the right if looking out from customs. It was around 11:00 Vegas time. Three security officers converged and were looking at one of their cell phones. A tall man in a suit popped out a door. Spoke with them and they all took off towards the far end. Past Carl Jr's. The tall man was riding cart with a siren on it. A few more security officers showed up and they all headed down that way. A plane had just let out and the people were not allowed to go down to baggage and were held in the main hall. I heard one of them say that all escalators, doors and maybe elevators were locked. The maintenance people doing the floors all disappeared as well and a little parade of security people headed that way. A few customs people came out and stood in the hallway. After about 10 minutes or maybe more a siren went off with an announcement of all clear. My curiosity got me wondering what the hell was going on?? Or is this normal in an airport.