Argus is a memory specialist and borderline indestructible because his body can just keep regenerating as long as it has fuel to burn. His item/clue could be located somewhere that it is hazardous for others to enter or require a memory probe to locate.

Rost being the head of magic R&D and highly versed in magical texts means his item or clue could be a specific book in the library, a spell component, or something from the lab.