For those unfamiliar with parallel construction (like I was) an intro:
Using thermal imaging to scan neighborhoods for the lights used to grow marijuana is considered a "search" by the courts and police have a warrant to do this. Let's say the police ignore this and go around secretly scanning cities without a warrant. They find out that John is growing weed in his basement, but they can't arrest him because this evidence is inadmissible. They follow John for a while and see that he interacts with Pete. Pete has a warrant out on him based on some unpaid parking tickets. So the cops arrest Pete, find weed, and get him to finger John as the source. Now the cops can get a warrant, search John's house, and "oh hey look there's all this marijuana growing here!"
This evidence is normally inadmissible as it was derived from an illegal search. It's very difficult to catch unless the police stupidly submit it to the court (as in the OP). Not using this tactic requires a high-degree of morality and/or oversight of police officers.
Using thermal imaging to scan neighborhoods for the lights used to grow marijuana is considered a "search" by the courts and police have a warrant to do this. Let's say the police ignore this and go around secretly scanning cities without a warrant. They find out that John is growing weed in his basement, but they can't arrest him because this evidence is inadmissible. They follow John for a while and see that he interacts with Pete. Pete has a warrant out on him based on some unpaid parking tickets. So the cops arrest Pete, find weed, and get him to finger John as the source. Now the cops can get a warrant, search John's house, and "oh hey look there's all this marijuana growing here!"
This evidence is normally inadmissible as it was derived from an illegal search. It's very difficult to catch unless the police stupidly submit it to the court (as in the OP). Not using this tactic requires a high-degree of morality and/or oversight of police officers.
http://en.wikipedia.org/wiki/Parallel_construction