Seems as though Britney Spears recorded a grip of tracks for her Circus album with bonus and exclusive tracks popping up all over the various different versions in each country. We can also tell you about 2 unreleased Spears tracks to be on the lookout for that were produced by Danja and written by The Clutch. They are:
"Abroad" and "Take The Bait" both with credits to Danja, 4 members of the Clutch, and Marcella. Hit us up if you found them!
Prop to Tom of The Keepers on the info.