Continuity mistake: During the car chase through the desert, the rear view mirrors on both cars disappear and reappear repeatedly.
Continuity mistake: The bullet hole fired into the red Cadillac by the bad guy later disappears.
Continuity mistake: In the opening scene, the hitmen's car loses a hubcap as it chases mobster Tony. In the following shots, however, all four of the car's hubcaps are back in place.
Continuity mistake: Throughout the desert car chase, the bad guys' Chrysler hood ornament and various hubcaps on different wheels of their car disappear and reappear multiple times. The car is dirty and then clean too.
Answer: Late one night, Geoffrey used a covert listening device to overhear David explaining his barricade device to Janet which is how he figured it out.
How do we know Geoffrey really did use a covert listening device? They didn't mention him using one.
There was a scene of him using it when David told Janet about his barricade.