Did you hear that the Paris terrorists left fake Syrian passports at the scene? Why would they do that?

I can imagine a couple of reasons.  1) False flag - to throw off or slow down any investigation that would lead authorities to search for the hime-grown terrorists, thus giving any survivors more time to go into hiding.  And 2) to cause even more negative reaction against the Syrians refugees, thus making the Europeans look bad to the Muslim world by throwing away women and orphans looking for asylum.