The bus was reported from Monarch Bus Company on Monday. A driver noticed the bus was missing that morning, but at first assumed another driver had taken it out early.
When the bus didn't return, company officials reviewed surveillance video and determined someone drove it out of the lot sometime after 11 p.m. Sunday.
Police said the bus must run for a while so its air brakes will work, so investigators believe whoever took the bus knows something about the vehicles. The flat-nose bus, which carries more than 70 passengers, is also difficult to drive.
Still, police found it Monday evening behind Point Plaza in Butler, about 30 miles north of the city.