Catan Seafarers discovery question
From p12 of the rules (emphasis added),


  If you place a ship or road that connects to the intersection of an unexplored hex, you must immediately draw the top hex from the pile and place it face up in the empty space.


If you can ever build a settlement on an intersection, you must already have a road/ship connected to that intersection, so all those hexes should already be explored; exploring does not require a road/ship to be built on one of the hex's edges.