Ghost Ship - S3-E4
Visible crew/equipment: MacGyver is repairing the broken boat to get to the abandoned ship. When he leaves the shore and starts to paddle towards the ship, you can see a boom mic drop into the picture from top left. (00:09:15)
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.
Trivia: As the series wore on, Dana Elcar began going blind as a result of glaucoma. This condition was eventually written into his character, as well.
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.