I had heard on the news that knew which room he was in because the smoke detector alarm went off and guest were calling about the sounds of gunfire. I can’t imagine they had to search floor to floor just to find him. Maybe to secure the perimeter, but he couldn’t have been that hard to locate, especially if the windows were gone.
If there was enough shooting to kill or wound that many by automatic fire, how could it possibly have been hard to pinpoint his location?
For their own safety they go floor by floor, the last thing you want to do is use the elevator, your dead as soon as that door opens or before. Floor to floor looking for possibly others waiting to ambush you on the way up. They had no idea how many were involved at the time is my guess.