Middle ground is still asymmetric. You know what they know about you, but you still know nothing about them. Your privacy is invaded, but theirs remains intact.

Yeah. I'm looking at it from a perspective of just making progress towards some sort of useful transparency. I would anticipate a lot less objections to what I proposed.

That wouldn't work. If the police are in the middle of an investigation, their lack of information about someone that's committed a crime is that evidence. There's a complex situation here where if a person knows they're being investigated, there's a higher chance they'll destroy the evidence that they're being investigated for. Yes, there are crimes around destroying evidence; but they can reasonably say "Oh, but I didn't check the police investigation database before shredding my old ledger", for example.

Assume the typical example is a record of a parking ticket or something (i.e., the police have thin or no records on average).

Assume that either checks of the database or certain types of additions to the database would have some period of latency (additions is probably the better way).

Police begin some investigation. For whatever reason, they don't want to reveal this investigation to particular potentially involved parties. Police go to judge and ask for 'order of leeway' or whatever it would be called. They get it. All it means is that the police don't have to publish ongoing details regarding a specific investigation. New traffic tickets go right in the database, checks of the database do not reveal the investigation.

Where's the problem? Take it one step further, is there a problem without a straightforward remedy?

Destruction of evidence is more interested in whether you are doing it to cover up something that you reasonably know is a crime than it is in what the police know about what you did, so I'm not real worried about it here.