The other day I made a post talking about how the map will glitch out and not fill in every now and then and that opening the map fixes it. PSA: Open the full area map every now and then
The image below is an example of what’s causing you to miss out on that 2-3% even though you explored everything.
As you can see in the image, I barely needed to get close to fill in that that very inconspicuous grey area. Had I opened my map as I was passing by, it would have filled in. There was no way for it to not fill in otherwise given that I explored the areas after it.
If you’re aiming for map completion, I recommend paying close attention to small spots like those you might have missed.