Interesting stuff in the CNN article on the conclusion of the Alabama kidnapping:

The FBI had borrowed from the U.S. military high-tech detection equipment similar to the technology used to discover homemade bombs in war zones, three Defense Department officials told CNN.
It was unclear whether the equipment, which is not readily available to civilian law enforcement, had been used by the FBI.
One of the defense officials said no members of the military were involved in the rescue. They would have been acting a technical advisers, the official said.

A commenter on another board suggested a more prosaic explanation: A camera hidden in the Hot Wheels car the FBI sent in for the boy. There is a precedent: The FBI sent a lot of bugs into the Branch Davidian compound in food boxes and other supplies, and that was 20 years ago.