someone built a device that DETECTS when FEDS is SPYING on your phone nearby
its called rayhunter, made by the EFF, and it costs $20
you buy a cheap mobile hotspot from amazon, flash it with their open source software, and carry it in your pocket
feds use devices called stingrays that pretend to be real cell towers so your phone connects to them instead, once connected they can track your EXACT location, grab your phone identity, and potentially intercept your calls and messages
counter-surveillance equipment used to cost THOUSANDS of $ and required serious technical knowledge
now its $20 and fits in your pocket