I've heard of people being caught pirating because of their isp monitoring their traffic. I've never heard of someone being retroactively accused of piracy because they owned unexplained mp3s. I'm sure it's never happened anywhere.
I don't know the laws in other countries, but in Poland owning unlicensed media is not a crime, only distribution is. That's why there are vulture litigators who sue you when you connect to a torrent network and start seeding, but they can't do anything about you downloading.
