Ok, what capability do they have?
Imagine all of the junk under any construction site. They barely detect things that are 2+ feet or smaller across under the soil. Big pipes are still marked and have geotag location because that stuff is useless. Nothing works from an airplane.
Handheld detectors will pick up pennies and old pop tops all day. What detector equipment is out there usually has a sending unit in the pipe.
Even a smart pig running sometimes loses connection and it is active and sending.
The sci-fi tech that you imagine doesn’t exist. We’d have it if it did. We are one of the biggest midstream companies in the USA, and Kinder Morgan would too, along with Shell, BP, ExxonMobil, and All of the other companies too.
Just bury your guns right next to the flagpole or metal fence vertical in a PVC tube.
Check out what Israel Keyes the psychopath murderer did. It’s impossible to know how many guns he hid.