[altern8tif]

Not entirely relevant to the original story, but Facebook does use machine learning to identify faces as well (similar technology to how Google or Apple uses in their Photo apps).

So even if you've never been tagged in a photo or checked into a location, there is a possibility that just by appearing in a random stranger's Facebook photo, they would be able to place you in the vicinity of said stranger at a particular time, and make more conclusions/connections from there.