[Genoa]

He could have left, then returned and temporarily parked outside and run back up

Very slight chance. Why park on the street, where you might not even be able to do it legally, then run up from the lobby? The garage would be quicker. Garage has elevator service, I assume.

[Cementjungle]

Very slight chance. Why park on the street, where you might not even be able to do it legally, then run up from the lobby? The garage would be quicker. Garage has elevator service, I assume.

Some parking garages are a pain to get into and out of... he could have parked in front of the main lobby in the 5-minute unloading zone and handed the attendant $5 and said "I forgot my phone in my room... be back in 2 minutes"

[Mr. Jeeves]

The garage would be quicker. Garage has elevator service, I assume.

The Mandalay Bay garage is a loooooong way from the hotel tower. No direct access. But the street is even further.