GEN:SID 1:2585
Message WEB-MISC nessus 2.x 404 probe
Summary This event is generated when an attempt is made to probe for a known
vulnerability on a web server or a web application resident on a web
server using Nessus.
Impact Information gathering.
Detailed Information This event is generated when an attempt is made to ascertain wether or
not a Web server or an application running on a web server is subject
to a possible vulnerability using the tool Nessus.

Many known vulnerabilities exist for each implementation and the
attack scenarios are legion.

Some applications do not perform stringent checks when validating the
credentials of a client host connecting to the services offered on a
host server. This can lead to unauthorized access and possibly escalated
privileges to that of the administrator. Data stored on the machine can
be compromised and trust relationships between the victim server and
other hosts can be exploited by the attacker.
Affected Systems All systems using a web server.
Attack Scenarios An attacker merely needs to use Nessus against a server.
Ease of Attack Simple.
Corrective Action Ensure the system is using an up to date version of the software and has
had all vendor supplied patches applied.

Check the host logfiles and application logs for signs of compromise.
Additional References  
Rule References nessus: 10386