Lessons not learned from the fingerprint database.
Something I remember hearing when these police über-databases were first mooted:
The retention of fingerprint data had increased over the preceding years, with innocent people, bystanders, etc stored. "Nothing to hide, nothing to fear."
This gave rise to two problems:
First up, fingerprinting isn't as accurate as most people believe, and one of the strengths of the old fingerprint database was that by only having criminals on it increased the accuracy, because the only people on it were ones likely to commit crimes (recidivism rates). Retaining non-criminal prints led to increased false matches.
Secondly, it turned out that the police used the fingerprint database as a list of suspects. As they weren't allowed to keep data on people charged-but-released, they had been considering anyone in the old database as "a criminal who hadn't been caught yet". They kept this attitude up after they started retaining non-criminal, non-suspect prints, and there were stories in the press about people who would get harassed after local crimes with no link to the crime or the victim other than living in the area... and being in the fingerprint database. Now imagine you have your house broken into: the police take you and your significant other's prints to eliminate them from the forensic search... and keep them on record. They never find the guy who did it, but every couple of years the police come knocking on your door because there's been a burglary in your neighbourhood and because they have no leads, they go mining the fingerprint database for "criminals who haven't been caught yet"....
The two lessons that should have been taken from that are that:
1) indiscriminate retention decreases the signal-to-noise ratio and increases the risk of false positives and (crucially) reduces the number of genuine positives.
2) the police always abuse databases.
But instead of tackling the root problem of the abuse, they just mitigated the symptoms slightly by reducing the set of innocent bystanders that are incorrectly identified by the police as "criminals who haven't been caught yet"....