This is a fascinating and in-depth description of how the US uncovered and reverse engineered sophisticated bugs placed in the electric typewriters of embassy staff in Moscow in the late 70s/early 80s.
Bruce Schneier suggests these devices may the worlds first keyloggers and it’s easy to see why. These astounding devices buffered characters typed by the golf-ball and sent them to the soviets using RF burst transmitters. It took 8 years to discover they were there!

https://www.nsa.gov/about/_files/cryptologic_heritage/center_crypt_history/publications/learning_from_the_enemy_the_gunman_project.pdf