Straight from the documentation:
http://publib.boulder.ibm.com/infoce.../r0022765.html
DB2® audit log ("db2audit.log")
* Operating system: All
* Default location:
o Windows®: Located in the $DB2PATH\instance_name\security directory
o Linux® and UNIX®: Located in the $HOME\sqllib\security directory, where $HOME is the instance owner's home directory
* Created when the db2audit facility is started.
* Contains audit records generated by the DB2 audit facility for a series of predefined database events.
Andy