The 90s needed actual spooks casing out pizza places... That was a real thing, Russians casing take out joints in DC, even before pizza was a big deal.
But it was easier just to keep an eye on the parking lots and how many lights stayed on overnight.
For at least the last decade, there have been electronic means of surveillance allowing every country who gives a shit to monitor 24/7. Which sounds stupid, but that monitoring is essentially free and just one of 100s if not thousands of individual flags.
Pizza flag may go off a lot, but if x, y, and z are all also happening...
Then everyone knows something is gonna happen.