I removed mine of I could install a mid pipe and when I stood the pipe up packing peanuts fell out of it, I looked inside and it's stuffed with packing peanuts lol, I guess that one way to refurbish a cat

 

no way, youd think they would melt

 

My initial though too, I noticed they were super white and there's ash and carbon build up in the exhaust so they couldn't have been there when it was running, what ever put them there didn't leave a trace on the peanuts, no marks at all I also didn't notice any droppings or anything that would point to a nest, but I think a rodent might have put them there my car has been sitting for little over 2 years