One thing I haven't seen discussed in this whole weird case: Supposedly the security guard went to check a stairwell door when it was opened. Thing about those fire doors--they lock one way. You cannot walk up the stairs to floor 11 and just open it. Apparently that door is silent alarmed to the security folks. Probably they all are.
But people go out the firedoors to the stairwell all the time. SOooo....the alarm was because someone propped it open. The security system is probably programmed to alarm if the door stays open for awhile, like a minute or two. Alarm mainly for liability--firedoors must automatically close, and continuous verification is smart. They get inspected by firemarshalls often enough to put silent verification systems in place, for a hotel with 1000 firedoors.
Someone opened the door, to let people in. Very common burglar method for teams. He was expecting company, and they may have been on the way when the security guard showed up. The rest of the team bailed when the dude shot up the hall.
Not true. A guy who goes under the twitter handle Nick Falco stayed in Mandalay Bay within the past several days.
He took his camera phone and took pics and videos of the elevators, the service elevators, the fire doors, the hallways, etc.
MB discovered what he was doing and a delegation of MB security plus an FBI agent went to his room and evicted him, and he was handed a trespassing warning by the hotel.
Anyway, what Nick Falco revealed was that fire doors are not alarmed and there are no cameras on the fire doors or in the stairwells. One can freely come and go up and down the stairwells and through the fire doors.
His twitter is now private, perhaps MB is putting legal pressure on him. But before when it was still open I watched the video he posted of himself walking in and out the fire doors into the stairwells.
He also discovered that there are cameras in the service elevators - so if Paddock used the service elevators to bring up his stuff, that would have been recorded, along with anyone who assisted him.