This app was mentioned in 7 comments, with an average of 4.86 upvotes
Sounds like "fog of war" in games, searching for that gave me the below for Android/Apple:
Sure, but I haven't been able to test it yet, I am getting my first android phone next week. But this seems to be in the spirit of what I was looking for. Wanderlust: Fog of War. According to the reviews it might not be optimal with the tracking/background and it haven't been updated for a while. (There is a free tryout version aswell. Wanderlust Free: Fog of War)
When I searched yesterday I think I saw some betas of other apps as well, but I didn't find them today.
Which one? There's like 500 apps named Wanderlust.
Edit: https://play.google.com/store/apps/details?id=com.rollingfortress.wanderlust
There's also a free (trial?) version, but it only covers 5 km^2.