[_delirium]

This issue that individual observations usually seen as benign become concerning when aggregated at computational scales is coming up in government surveillance as well. It is fairly uncontroversial that the police reading your license plate as they walk by your car isn't a 4th-amendment violation: it's not a "search" to view the exterior of a car parked on a public street. But is it still okay if the police start tracking the movements of all cars around the city using a network of license plate readers? Many people would be wary of that, but it's not clear what the difference in kind is or where a line could be drawn.

In the government case, the "mosaic theory of the 4th amendment" is an attempt to produce a new rule that certain aggregations of non-search data become searches when aggregated, so it may require a warrant to track you around the city with a network of license-plate readers, while it doesn't require a warrant to look at the exterior of your car normally. It's a bit fuzzy, though, and not clear it will survive (or be practical to administer). And it's not clear whether something analogous should or could be done for private collectors of data. And of course if it isn't, then that's an easy loophole for the government too: the government can just buy the data from the private data collectors. Under current law, at least, the government buying data lawfully collected by a private party who willingly sells it isn't considered a search, and doesn't require a warrant.