Jack of Lies - S2-E6
Continuity mistake: When MacGyver et al. spot the car they will use to escape, the headlights are clean and white. After the commercial break, they are much dirtier and the dirt is only on certain precise spots on the headlights. (00:33:15 - 00:35:25)
Continuity mistake: When Tommy is escaping into the mountains to try and help, a small bit of his jacket tears off onto a tree branch. MacGyver later sees it, but the branch that the torn piece is on looks entirely different. (00:21:10 - 00:22:10)
Visible crew/equipment: Early in the episode a thumb/finger is visible at the bottom of the screen turning the timer wheel simulating a working timer that is actually just a dummy prop, a few seconds before it explodes. (00:02:39)
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.