Haha. This would be very usefull for the blue team! I kinda like your idea. But only if the detectors only can sense within a small range and are rare.

What do you guys think about a way to detect enemy "seismic activity"? Basically I guess the map would just show waves where enemies are digging or throwing grenades. I suppose either some placeable seismometer with a range could be used to detect this or it could just be detected anywhere.

I know it could change the gameplay, but personally I prefer surface games over tunnel games, which feel very unpredictable to me. In reality I know it wouldn't stop tunnels from being made, but at least with this feature you could tell when the enemy breaks through some blockade you made over the tunnel.