I'm picturing a custom made compartment with a remotely activated release switch. Not the hidden map compartment in your console or the hidden compartments that are under the rear seats of most trucks. That being said, I'm sure if there were drugs in there at some point there would be some sort of residue. But like mpl006 said above, what would happen if he bought the car and the compartment was already in there. As always, I'm sure there is more to the story.
My guess is the guy was already caught or suspected of doing something bad. Cops don't just start looking for hidden compartments on your car without a good reason. I agree...there is probably much more to the story.
--Sent From My Galaxy S4