When Sam Casey activates his atomic watch it makes him vanish, clothes and all, but when dealing with hijackers we see the gun he is holding, while he is invisible, but the gun should be within the invisibility field thus invisible as well.
Dr. Hale is pretending to be in the truck that Sam is driving. He's overseeing Sam's progress via helicopter and whenever contacting him, the noise from the rotor blades would be heard in the background yet Sam can't seem to hear it and assumes the Doctor is still in the back of the truck.