Jack of Lies - S2-E6
Audio problem: At night when MacGyver and Jack are sneaking past the guards behind a mule, we see about 8 to 10 light bulbs hanging overhead. MacGyver later pulls the cord down and all the lights begin to pop. There are about 16 pops, many more than there were bulbs on the cord. (00:24:50)
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.