Since the 29 Nov 2017 Patch Notes in EU/NA these now have a chance to drop while fishing in certain waters (Margoria, Ross Sea, Velia hotspot, Grana and Lake Flonder), like Silver Keys.

Give 100 Silver Keys to Morco in Altinova

Now every Morco give it.

I just got one from a Wild Herb node on Coastal Cliff (near road on top). It appeared in the item list window as a pile of dirt and got a message that something was buried nearby. I dig it up and it says it wants a gold key which I don't have but my other character does so I switched over to see. It disappears when you do that and when I switched back to the original character it was gone as well.

Any idea on how to get these?

I would guess we get it with the Ship update soon, maybe in Korea too.
So keep your silver keys to change them 100 in 1 Gold key.