I'll try to help. The game usually count as "explored" any area that has been at 500 meters from your position at any point.

Thus, grey areas that you have not gone into: safe. Areas where you have only sailed around the coastline not entering the terrain: if they are big enough, then there should be safe areas where they can appear. Keep in mind that this also depends upon the seed of your world.

In worst case scenario, do remember that the Deep North areas have some Mountain biomes interspersed as well. Depending on the seed, there may be some caves there.

Later, when the Mistland updates happen, the same mechanism will probably be used so do leave some of those coastlines unexplored for later.

Best of lucks, may your axe bath in the blood of your enemies and your drink flow plentiful!