Tibs, the "hidden" network is appearing and disappearing due to the way radio waves propagate. Sometimes conditions are just right for you to be able to see them, maybe a car going by reflecting the signal, or the way tree branches are moving in the wind. Tons of variables effect this.
I use a program called "Net Surveyor" and it shows networks as far away as almost a half mile. The signal levels are far below anything useful, but the networks can be seen. Sometimes they are there, sometimes not, again depending on the specific signal conditions when I look for networks. I can always see the neighbors networks on both sides and across the street, easily a few hundred feet, and usually see the one two houses away. I wouldn't be concerned, you don't have a nefarious Russian spy in the area.