Guys,

This is an article with a lot of good info to read and learn from!!!

Not every case of a successful intrusion is “crowned” with a replaced Web site on the server, data theft or damage. Often electronic intruders do not wish to create a spectacle but prefer to avoid fame by hiding their presence on compromised systems, sometimes leaving certain unexpected things. They use sophisticated techniques to install specific “malware” (backdoors) to let them in again later with full control and in secret...
Source here and inside you'll have to click where it says: "Click here to read the full story" (and please click it because inside that link is the real good article full of tools and links).

Enjoy your reading!!!

DKRR