I seem to remember an old jailbreak app for iPhones, called Signal I think, that triangulated positions of the cell towers you were connected to and plotted them on a map. I wonder if something like this could be used in an app, to warn people when a stingray was capturing their signal. If your app "remembers" the positions of towers, and it suddenly sees a new one, or it sees one that is not stationary, seems to me that'd be a good sign that something wasn't right. Is this possible, or am I misremembering?
Even better would be if the app connected with others to create a crowd-sourced database of where and when they are used.