Excellent question... did he figure that they were about to see the cameras on the service cart and they would be beating on his door anyway trying to find out what was going on? The question goes hand in hand with the one of why did he stop shooting after 10 minutes when he had lots of ammo left? And why did he arrive in LV with tons more ammo and other materials left in his vehicle that never got used?
IIRC, the story talked about Paddock having installed some kind of bar across the door, which likely set off the door alarm, hence the guard/maintenance guy. Paddock may have thought they were going to see the bar on the door, which triggered the start. Too many moving pieces to be convinced, though . . .