Topography + weather+ cellphones = cheap sound detection network, non detectable by harm (yet)
As in high-speed anti-radiation missile:
<https://en.wikipedia.org/wiki/Anti-radiation_missile>
I don't know how vulnerable counter-battery radar is to radiation-seeking missile attacks, though suspect that's a risk. Wikipedia mentions the risk, and links a reference for further information, though I'm not finding information there based on a quick skim:
<https://en.wikipedia.org/wiki/Counter-battery_radar>
<https://web.archive.org/web/20141226051243/http://armypubs.a...> (PDF)