Continuity mistake: Here as in every episode where MacGyver's Jeep appears, it's the '80s model with the square headlights. When we see him fixing the Jeep after the wreck, the headlights (or at least the holes left by them) are the older round ones.
Continuity mistake: In the scene where the gang are chasing after the two kids, it is raining and the street is wet. They stop chasing and turn around when they see a police car go by. In the first shot of the police car, the street is completely dry. (00:05:45)
Continuity mistake: The episode begins with MacGyver's Jeep having a blowout and going off the road, however, when we see the overturned Jeep, all 4 tires are inflated. (00:01:45 - 00:02:15)
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.