That said, I'm abstaining on this vote because I didn't perceive this as a "demand the explorers return what they found immediately" choice, just a "try to get information on the source" choice, so I feel like this is not what I wanted anyway.
Were you planning on figuring out who they belonged to, and then keeping them anyway?
You back down on the suggestion that the treasures be returned to their original owners, satisfying the explorers. The expedition leader closes out by pointing out that while maintaining good relations with the natives may be important, he's pretty sure that nobody has cared about these treasures for many years, and that nobody will miss them. He may have a point.
However, it's still possible that the ruins have some significance to the natives, so you make a point of inquiring with your allies about them (without mentioning the treasure). You ask your wife, but she doesn't know anything- though she does mention that the high shaman of her father's court sometimes told stories about weird things like that. Consequently, you send a messenger to the Stone-Axe Tribe, asking them for information. You also send a messenger to try and contact the Hide-Shield Tribe, who supposedly inhabit the territory the ruins were actually found in- but they are a very reclusive people, to the point where you have only heard about them from the colonists, and never seen one in Nova Emperia.
Before the messengers can return, a ship arrives. It brings a cargo of trade goods (tea, spices, and liquor), as well as the usual supplies, a handful of immigrants, and... the Imperial Envoy.
The Envoy explains that he has been dispatched to audit the colony; he requests (or rather, demands) access to all records, and the ability to question anyone- including you.
A: I have nothing to hide; anything you need, I will provide.
B: I resent the implication here. I shouldn't need to cooperate to prove my innocence.
C: Oh, that sounds like a lot of work on your part. Why don't you just relax, and take this chest of unmarked non-consecutive silver ingots instead?
TURNTURNTURN