I installed an osc contribution which directs 404 errors etc to a clean page that looks like one of the sites pages, with a continue button. It also sends an email to the store owner with details of the error. This has proved to be great at finding missing images, etc, but at least once a day (usually more) I get:
All three come from the same server at the same time. As per Michaels advice a while ago (and now on 2.5 install) I renamed the admin directory to another name (I use 2.0.25). I think it shows that hackers are alive and well and continue to try to hack osc sites!