From Team Fortress Wiki
"Intruder Alert" is one of the song titles featured from the official Team Fortress 2 soundtrack, listed as track number eight. Added in the WAR! Update, it is one of the three songs featured in Meet the Spy, as well as being one of 18 main menu start up themes. It is the fifth track to be added via patch subsequent to the game's release.
December 17, 2009 Patch (WAR! Update)
- This song was added as main menu music.
May 9, 2011 Patch
- [Undocumented] Added an 'Intruder Alert' midi file.