I have two close friends who have “gone dark” in the past couple years. They got rid of their cell phone accounts and started using burners. Cancelled their cable, internet, and landline services. Quit social media (including one of them on FR).
Both subsequently had visits from the FBI.
One was asked why he did this and he told the two polite FBI guys, “Because I suspected I was being monitored and now here you are!”
The FBI guys laughed, wished him a nice day and then left.
"Burner" phones are readily identified by automatic means and get singled out for special attention.
Agencies can track them in real-time and track a log of their movements and locations retrospectively for days or weeks. True as well of all cell phones, but easier with "burners".
If you "solemnly swear that you are up to no good", you had best not be carrying any cell phones.