When I was a Air Traffic Controller law enforcement wanted us to call them anytime a certain Navajo taxied for takeoff. Yea right, we never called. ATC doesn't do things like that.
A couple of weeks later the feds had a guy in the tower watching the Navajo. After a few days we let the fed drink our coffee. No love lost here but he was bringing donuts every morning. We tolerated the guy, even spoke to him sometimes. Finally the Navajo departed and we got a radar ID on him. Handed the target off to center and the fed left.
A couple of days later the fed is back in the tower and the Navajo is back on the ramp. Appears the Navajo knew he was being watched and flew down the canyons below radar.
The feds decided to install a 2nd hidden transponder in the aircraft set to a special law enforcement code. In the middle of the first attempt the guy came to the airport and almost caught them. In the tower we could see both sides of the fence. At one point they were 50' apart with a block wall in the middle. Someone was screaming abort, abort on the radio. The feds were trying to get the inspection plate on and kept dropping screws. It was so frickin funny I could hardly work traffic.
Attempt 2 with the super secret transponder was made at night. Success!!! A day or two later the Navajo departed and we thought we'd seen the last of the fed.
About a week later the Navajo was back on the ramp. I was working traffic and the fed came up the stairs with his usual peace offering, a box of donuts. I just looked and shrugged. The fed said,
"The guy found our transponder. He mailed it back to us."
Frank Holbert
Flying the Bellanca Super Viking