'Full-Pipe' FBI Internet Monitoring Questionably Legal
This is where it is sorta tricky. This part has been interpereted to say that anytihng found durign a reasonable execution (aka, no searchign in pill bottles for stollen TVs) of a warrent is admissable.
The rule for physical searches is "in plain sight".
If the police are searching your home for, say, a fugitive, and you have a bong on your table, that's in plain sight.
Data mining is the exact opposite. It's taking a microscope to any minute detail you can find, but strip-searching not just an individual home, but an entire neighborhood. That's the kind of physical search that's explicitly prohibited.
Does the principle apply to wiretaps? You'd think so.