What if they hire some NPCs to go get it? What if they convince the King that the treasure is a Royal heirloom and the Kong sends an army to retrieve it? What if the PCs just sell that information to the highest bidder?Why? It is how Linear Games work.
Like I said, a Pure Sandbox is just chaos. The players simply do random things as the DM-player says "yes" can creates the world right in front of the characters.
That is why few sandbox games are pure. As soon as a play sets a goal and detail is added to the game...it becomes more Linear.
The treasure is at location Q2, so the characters must go to location Q2 to get the treasure....that is a Linear Action.
My point is that the fact of the treasure at the location has no bearing on linearity vs openness. If the campaign ground rules allow the PCs to engage that fact however they want (including ignoring it) then it's a sandbox.