I am currently testing about 20 plagiarism detection systems (PDS). During one of the tests we saw a very nice turn of phrase that we had plagiarized from one newspaper site (with permission!): "paranoide vorolympische Kraftmeierei" (paranoid pre-olympic muscle-flexing).
The system did not find our source, the Süddeutsche Zeitung, but the Swiss Tagesanzeiger. Putting just these three words into Google proved something I have been saying all along: three to five words suffice.
As it happens, the author, Henrik Bork, is the author of these identical articles. He sold one in March, one in April. The ethics of this is another discussion. But the use of PDS is so time-consuming, one really just needs to pick out phrases like this upon reading, and use a search engine. Full stop.