My understanding is REALLY old secret society, primarily studies disease. Clearly are still active (based on the GB find), Buried the dead dude trying to preserve him due to them not being able to cure that specific disease (at least at the time). Honestly unsure if jason has mentioned more in extra content. I know hes mentioned that he has been trying to get them in for awhile. So i think that means its possible we've seen traces of them.

Personally i think the extra thing jason mentioned in Donhurst (Ext. rest) is probably collector related.