Continuity mistake: During the car chase through the desert, the rear view mirrors on both cars disappear and reappear repeatedly.

More mistakes in MacGyver More quotes from MacGyverMore trivia for MacGyver
The Eraser - S2-E2
MacGyver: Dreams are important, otherwise sleep is just eight hours of nothing.
More questions & answers from MacGyver





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.