You say: Our rules that allow criminals to walk free when police do improver evidence gathering and surveillance to catch them are there in part to keep the police from use of those powers on the innocent. Almost. The rules are the to keep the police from misusing those powers on the innocent. But you haven't outlined the misuse scenario. And I'm not sure you can -- at least, I can't see what the problem might be. Imagine a case where you have, say, security cam pictures of a crime, and you canvass neighbors of the crime scene for information about who's in the photo. I don't think there's anything wrong there. Similarly, canvassing a DNA database for for phylogenetic neighbors of a given sample strikes me as innocuous, assuming, of course, perfect reliability of the technical processes involved.