A representative of a German university asked for my help this past week. They want to purchase plagiarism detection software, but their legal department insists that they can only purchase software that they run locally, they may not send the papers to a third-party company for testing, as the papers are examination artefacts and not to be used outside of the university.
This gives plagiarism detection an interesting twist: software that runs locally does not normally have its own database - so it basically is just doing the search machine searches for you, in which case you might as well be doing the testing yourself. It is conceivable that a university might start a papers database of the locally submitted papers, but that will only be of marginal use, as copying from the Internet would not be found.
I have heard that locally installed plagiarism detection software has trouble negotiating licenses with large search machine companies for fast, repeated searches. So maybe what we need is some sort of Plagiarism Workbench that helps teachers do their searches themselves, recording what they tested when and helping them do documentation.
But it seems there is no substitute for doing one's own searching. Since we are, one hopes, actually reading all the papers and not just assigning random grades, we might as well do a quick check after reading on a few paragraphs. As I have often shown: 3-5 nouns suffice.