Mystery Insulation Inside HVAC Duct

[Steve K.]

Working on a flip and came across a head-scratcher...  when cleaning the ducts I found a few softball-sized chunks of yellow fiberglass batt insulation INSIDE the ducts. I figured maybe it had fallen in during construction or a kid dropped it through the floor register or something. I didn't think much of it at the time. However, I continue to find more pieces of insulation appearing in the ductwork several months later. 

I inspected every accessible part of the duct system, cleaned them several times (they were really gross) and scoped with a camera a few times but couldn't find any holes or gaps or anywhere that it could be getting sucked in or falling in. My HVAC guy and several other experienced contractors could not figure it out either. 

The house was a complete mess: rodent infestation, mechanicals shot, original 1989 tract-shack. We did a full-gut remodel and addition, took care of the rodents, re-insulated when the walls were open (almost all the fiberglass is out now) and replaced the furnace and central air plus added a few air returns to the HVAC system (there were only 2 for a 5 bedroom house). 

I can't figure out why I keep finding insulation in the ducts. Has anybody else seen this before or have any idea what is going on? I'm stumped.