The concept of the untitled bonus track is similar, yet different to that of hidden bonus tracks.
As described in the node of the same name, hidden bonus tracks are tacked on to the end of a song usually 5 to 10 minutes after the song has ended. Untitled bonus tracks are different. They are just that, a bonus track that you don't know about from looking at the case, but you see when you put it into the CD player or computer.
"Wait... this CD has 12 tracks listed on the case. Why are there 13 showing up on my CD Player?" is a typical phrase heard when the bonus track is discovered.
Then there's the case of later copies of the "Broken" EP by Nine Inch Nails. There were 6 tracks listed, then 91 tracks of about a second each, and a song on track 98 and 99. Those could be classified as untitled bonus tracks, but I just call them annoying. The first issue of this EP had those tracks on a 3" CD that was enclosed with the digipak.