As a note, I do not believe in what Im about to type.

You could use consistent and unique biotrackers, distributed to the same place and time. Meaning that anytime you detect that biotracker, it necessarily means they were in that place of distribution at a certain time. Over months of tracking this way, you have almost certainly enough data of locations and times to crossreference that only one or maybe a handful of individuals to satisfy.

What is the point of this? I dunno.