Fan Casting Bluff the Eavesdropper in Sort by Tropes

People notice their phone is tapped, or see a microphone in their potted plant, or otherwise discover that someone is spying on them. So they start faking. A specific version of this is when two people being audio bugged carry on a conversation for the benefit of the bug while holding the real conversation by sign language or exchanging written notes. (And the visual conversation will often begin with a note or signal meaning "I should warn you we're being bugged".)