More on Allonges

My post on allonges generated quite a bit of response, on and off the blog. But here's what's troubling me--if allonges are being fabricated ex post, who is doing it?

• The servicer?
• The default management shop (e.g., LPS)?
• The network firm (sort of the foreclosure GC)?
• Local counsel? 

I'm curious to hear any insights anyone has on where the document production is taking place. 

I'm also curious to hear insights on whether the production of allonges is because they simply do not exist or because they are being held by document custodians and for whatever reason (cost, trust) they aren't getting down to local counsel. In other words, is this outright fabrication or simply recreating documents that actually exist somewhere but aren't moving down to local counsel?