MxAngel - 8:40 pm on Jul 30, 2012 (gmt 0)
It's most likely a troublemaker trying to probe your 404 response.
At one stage two IP's were listed on Project Honey Pot. Do the IP's have other requests?
For example on my site I see some of them trying to exploit the timthumb vulnerability and then they end with a GET /undefined
Also there's a little bit of more information in the following topics: