Bill Zebub
“It’s probably Matt Mercer’s fault.”
I use something similar to this when I play in person. What I do is add small dots onto the grid to help me for when I draw in the rest of the map. I, fortunately, have not run into the scenario where a player looks at the map and (in)correctly guesses that there must be something there because of the blank space. But I'm just lucky.
Part of that is the style of map. The one I will be using will tend to make blank areas obvious, unfortunately.