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.
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.