Starter question: visitor or visitors? Same IP, same UA, same intangible-something?
Another possibility is that they're looking for an unwanted directory that might have been installed by a previous malign robot, like the occasional requests for /muieblackcat*. (btw, does anyone know what this directory, if it actually existed, would contain? Other than bad news, that is.)
* "Why yes, kind visitor, it's located over here in the 127.0.0.1 room. Make yourself at home."
So far it's just the one visitor - yesterday. The UA was standard: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB7.1; InfoPath.1; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 1.1.4322; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729).
They came to the site directly without any referral string etc. and didn't visit any real pages.
A Google for "___possible__unsafe__site__" [google.co.uk...] does show it's popping up quite a lot!
I also contacted the OP in the previous WebmasterWorld post but he didn't find out what it was.
I noticed this too, all from one IP address in the 93.107.104.* range, a hitting a few other made-up pages as well. It's probably a bot looking for vulnerabilities or places to spam, that's my best guess.