I would think the ears are the most likely part of a Grabrat to remain behind after combusting. If you could bring me 25 of these, I could see how similar they are to common rat anatomy.
I'd like to study one of the Mimics from Steamport City next, and something that more closely resembles a native creature. Grabrats seem to be physically similar to common rats, even though their behavior is completely different, and they're obviously a lot bigger. I could use any remains that are left by combusting Grabrats for my research.