MacGyver: I'm not trying to be mysterious about it, it's just that what I do is tough to explain.
The Eraser - S2-E2
MacGyver: Dreams are important, otherwise sleep is just eight hours of nothing.
The Eraser - S2-E2
MacGyver: Look, if this works, it'll keep us from gettin' caught. If it doesn't, it'll keep us from gettin' old.
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.