Note that sharing was the issue again. It always is.

Actually, showing up for a moment as a potential source for
the file would automatically trigger an additional search
through the same network. That's just normal police
procedure which doesn't require a warrant or even any form
of personal contact. But not showing up again as a source
and not being findable in a search for the same file and not
showing any suspect file in a browse is always going to
result in the same thing, being dropped for probable
suspects who do show up on the search and stay there.

The key concepts here are reasonable suspicion
and probable cause.

Reasonable suspicion is grounds for further investigation
but not a warrant, and only if that further investigation
brings up probable cause can any further action be taken.
It's as simple as that, and there's no getting around it. It's
the law everywhere in Europe, North America and all other
countries with a constitutional government and a highly
developed legal system. Any experienced police officer or
prosecutor will confirm it if you go and ask. This legal
principle goes right back to the days when modern law as
we know it was first brought forth and put into force.

There would never and could never be a warrant issued for
a single hit on a single download attempt and police would
never stop checking to avoid not finding corroborating
evidence just so they could beg for a warrant based on one
possible hit.

Show us one person in the whole planet who was ever even
questioned for an accidental download which they then deleted.
Just one will do. How many years did that one get?

Never, ever forget: The only thing that will get any
movement at all is a definite indication of sharing of such
files. That means documented verifiable proof that can be
brought to court. A possible indication of a download
attempt which may or may not have failed is not enough if
further investigation yields nothing. This is particularly true
when investigators already have plenty of probable cause
on dozens or even hundreds of other people to work on, and
we already know is happening everywhere because of reports
in the news on the topic.

Clinging to an untested theory for the sake of argument is
not helping those few people who come here for info about
what to do with accidental downloads. If you know
anybody in law enforcement or the courts ask them about
the distinction between reasonable suspicion and probable
cause, or search the web for articles that mention both
terms with that specific wording. Once this central and
basic concept of the law is made clear you will understand
a lot about how the law, the courts and the police work
together to protect the rights and privacy of people like you
and your fears will be somewhat relieved.