Marilou flew out the 25th, so, maybe she took the airport shuttle from the hotel and left her car with him? or he gambled after he dropped her off? We’d need to know if Paddock checked in for the room service receipt to be credible.
Mandalay Bay does not offer a shuttle service. Shuttles are available at the airport and they drop off at all the major strip hotels. Mandalay is usually either the very first drop off or the very last drop off. Shuttles generally cost $7-$8 per person.
https://m.hyatt.com/mt/mandalaybay.hyatt.com/en/hotel/our-hotel.html