[analog31]

What if civilizations go dark after a century or two of rapid technological progress. Look at what we're doing. The bulk of our data bandwidth is now confined to fiber optics, that leak virtually nothing to outer space. Remaining wireless comms interfere with one another, using ever decreasing power, and are designed to be hard to distinguish from noise. It's like we've enclosed ourselves in a primitive Dyson sphere.

A faint shift in the molecular spectrum of our atmosphere might be telling.