The Kopp-Etchells effect is rather poorly documented. It's only just been named during the Iraq/Afghanistan wars, and I haven't found too many research papers on it to date.
Actually, the only thing that wasn't stolen at the Louvre's Apollo Gallery is the Regent Diamond, figure no one would take it because something to do with a curse.
I've also found that the two thieves were leaving for Algeria, thanks of a trace of DNA evidence recovered from a helmet and their past in burglary cases according to the police.