Bieber, 19, was not on the bus Sunday, the Detroit Free Press reported. Kenneth Hammond, a spokesman for U.S. Customs and Border Protection in Detroit, said the bus was searched after a dog alerted on drugs.
The bus was heading from Windsor, Ontario, to Detroit via the Ambassador Bridge. Bieber performed Sunday at the Joe Louis Arena in Detroit.
In March, marijuana was found on a Rihanna tour bus at the Ambassador Bridge. Like Bieber, Rihanna was elsewhere at the time.