While playing Sea of Theives, my friends and I saw a sloop on a nearby island, and wanted to go say "hi".
He didn't see us approach, he was on the island doing a quest. I jumped on his boat and stole the most expensive looking thing I could find, which was a named chest that contained a skull with blue crystals coming out the top. We left without sinking his ship.
My friends told me we couldn't actually sell the skull, it was purely for quest purposes. So we thought at least it would look better as a decoration on our mast for the session rather than as a completed quest for that guy.
But then the skull disappeared and we were all sad, but the empty chest remained on our boat.
We hypothesized that the guy signed out and the quest item went with him. Could that be the case?