I got US RETRO w/BONUS DISC - Try & figure this one out!!!

After returning my pre-ordered Amazon.com copy of RETRO because of no "extra tracks", I went to my closest record store (Newbury Comics) and there was one copy left of RETRO with the 5th disc taped to the front of the shrinkwrap, for $10 less than the Amazon.com copy. At first I thought it was surely the UK import, but saw nothing on the packaging indicating this was anything but the domestic release with the disc attached. Several store employees and the manager confirmed this was indeed NOT an import. So where did the bonus disc come from? No one at the store could tell me and everything I've read says Rhino isn't releasing the set with bonus for a while yet. Although I was thrilled to pick up a complete 5-disc set so easily, where did this thing come from?