I know, sometimes i had that feeling too. I wanted to make it visible with some dark-gray color, at least the zone where you've been...
... but the fact is that the code dont permit that because i worked with map tiles (0 - signal, 1 - signal). The map i think have 100x100 (later edit 300x300), so it's not suitable for coloring (a rounded coloring).. only shrap square edge. (i think i'm gonna do that)
I had another map generator - with random signal (position, range) .. but the problem was the testing algorithm. The fillout algorithm doesnt work for this kind of maps. I did't knew what algorithm should write, so i used the most common and the easiest one :)