Wow, sorry for your loss. The photo of the secretions looks very suspicious! IMO. I would agree with Citrus that it could be a parasitic entity or a gastrointestinal disease of some type. It would be a good idea to bag and tag a sample of the brown fluid for testing at the vets office. And definitely deep clean your other animals enclosure and treat this as very contagious. Maybe its not but treat it like it is until proven otherwise. Good luck.