I have installed the w3 Total Cache for some of my sites and apparently a few of the sites get hacked with injection of unknown php code which is presented in this Support Thread
http://wordpress.org/support/topic/wordpress-virus-4?replies=7
This w3-total-cache/wp-content/advanced-cache.php file gets overwritten with that unknown code and a copy of the advanced-cache php gets copied to the the root wp-content folder as well...
I have re-installed and cleared the code many times but it just keep coming back... so wondering if there is a security issue with the plugin.