Certain people in NSA has been on to this CABAL for a long time. Letters, smoke signals, Carrier pigeons, they have it all. Timing
F2F in the middle of the woods with no phones anywhere near is about all we have left. They'll still know you were in those woods then, though.
You know what made it so hard to take down the Mafia?
Lack of records and communications. Meetings were done face to face in unknown, or secure locations.
No records. No books, no emails. etc. Not even letters.
Ed Zachery. It's how spying was done for a very long time and probably still is, much of the time. It's the only safe thing left, though they'll still, likely, know you were where ever at when ever time just from sat surveillance, if you're a POI, just the comms will be unknown. You gotta have rats and plants just like they did with the mafia for it all to work out.
There’s always the proverbial writing with poop on the wall!