Apparently some rightsholders are using the BitTorrent DHT (distributed hash table) to find infringing users. This is terrible practice, because a BitTorrent client, simply by running (even if it has never downloaded or uploaded anything, let alone anything infringing), participates in the DHT and passes on information about which peer to get files from - and some of those files may be infringing. A rightsholder using this method can't distinguish between clients simply participating in the DHT, and those actually sharing the file.

Relevant comment from 3strikesNZ's Facebook page:

3strikesNZ: Couple of months ago, InternetNZ got an email from CBS saying we had illegally downloaded an episode of The Good Wife. On investigation we found P2P software running on a test server. It had got a request for that TV show, didn't have it (in fact it didn't have anything on it) but helpfully pointed the ping to others. This was interpreted as us having the file and would have resulted in a notice under the new law.