The metal detectors, which were to have been used at a campaign event for Vice President Joe Biden, were found at an abandoned house in Detroit and in the U-Haul, left near Henry Ford Hospital, the source said.
Surveillance photos from security cameras at the hotel show a man in a green T-shirt and dark shorts near the truck before it was taken.
The stolen truck was found Monday, Secret Service spokesman Ed Donovan said.
The law enforcement official, who did not speak for attribution, said there were no weapons inside the vehicle.
Biden spoke at a downtown rally organized by the Michigan AFL-CIO.