NEW SPRINGVILLE, Staten Island (WABC) -- A suspicious device was discovered beneath a city bus at the Yukon Bus Depot in New Springville, Staten Island.Who may have placed it there and why are still mysteries."We're not sure. And I'm not even sure if the police are 100% sure of what they have. They said a tracking device. But today you have to be careful," said Daniel Cassella, President ATU 726.On Wednesday just past 12:30 p.m.at Staten Island's Yukon bus depot, the NYPD responded to a call of a "suspicious package".The bus, union officials say, had been in the depot for a routine maintenance inspection.The device was attached with a large magnet, Eyewitness News has learned, to the bottom of the oil pan near the bumper."One of the mechanics spotted it and knew it shouldn't have been there," Cassella said.The NYPD revealed in a statement: "A suspicious tracking device was discovered under an MTA bus."The depot was evacuated for about two hours. No more devices were found."Our buses are inspected about once a month. They are gone over top to bottom, mechanically and if we see something unusual it's going to be caught," said George Andresen, mechanic.Even neighbors were not sure what was happening."They didn't know what it was. They were waiting for the bomb squad to come. I was like wow! Nobody said anything. God forbid there is a bomb right here they should have let us know," said Peggy Moran, a neighbor.An MTA spokesperson said, "We are not commenting. It is still an active investigation."Still, drivers are concerned."We just try the best we can to inspect our buses and make sure things are right for us as well as our passengers and customers who ride," said Dion Hickman, a driver.All 800 buses on Staten Island were inspected with no other devices found.The buses also are equipped with GPS so the MTA knows where they are at all times, leaving union officials to only speculate why the device was placed on the bus."I wish I knew because it would soothe the minds of my members also and the public, but at this time it would just be a guess," Cassella said.