Proof of concept
This project proves that it is possible to continuously track the movements of individual members of a population over a long period of time.
Jaowon, you're almost there.
The anonymized data establishes the base line of average dwell times. What if a non-coffee shop area of interest is defined on the map, with attached reporting criteria when a number deviates from the base line? There would still be a lot of data to wade through, but it is all already conveniently packaged for automated analysis. Now you know who to put in the database for terrorist proclivities. So much easier than spotting Transformer's T-shirts.
You could even establish criteria that automatically defined new points of interest: cross off all areas of congregation that map to already known venues, and you're left with all the new terrorist recruiting stations (or meeting places of the political opponents of the powers that be, but surely it would never be used for something like that...)