Sam halted as well, watching for any other moving objects, or any hidden mechanisms that might be moving them--on the off chance that it was just a prank. At least the book was floating, and not being used as a flying projectile.
He didn't call out, like they did on those Ghost Facer type shows. If there was someone there, it more than likely already knew they were watching, and it would reveal itself when it was good and ready.