As often is the case these days, I was using Exact Audio Copy to test a CD before sending it out to another La La user, and notice that track one is highlighted in red. Curious to find out why the program has chosen to highlight the track this way, I then found out about Track Zero, a type of hidden track I had never heard of before.
Most of the time tracks are hidden at the end of CDs, but there is also a way to hide them at the beginning of the CD before Track one starts. The only way to hear them is to put them in a regular CD player and when track one starts playing hit the rewind button until it stops. Not all CD players can rewind in this manner including computer drives.
The CD I found it on was the self titled I Am Kloot album, great track by the way....
Other CD with hidden Track Zeros include
Songs In The Key Of X: Music From And Inspired By The X-Files, there are two hidden tracks;
Factory Showroom by They Might Be Giants.
Super Furry Animals - Guerrilla
Ash - 1977 (two hidden tracks apparently)
Calexico - Feast Of Wire
Queens of the Stone Age- Songs for the Deaf, track entitled "The Real Song For The Deaf"
There are more but this a good list to start with....