From MTV:
According to Swedish paper Aftonbladet a spokesperson for the Stockholm police department confirmed that around 7:10 p.m. local time an officer detected the smell of pot around Bieber’s bus, which was parked outside of the Grand Hotel. After the bus left the hotel on its way to the singer’s show at the Globe Arena, the officer contacted the narcotics unit, which issued a warrant to search the vehicle.
“A colleague felt a strong smell of marijuana, like someone had been smoking in the bus,” a press officer for the Stockholm PD told the paper.
An hour after Bieber took the stage, police searched the empty bus in the parking garage and found a small amount of narcotics, which local papers reported was marijuana, as well as a taser. At press time, no suspect has been identified yet and the bus was not impounded.