There's the other end of a technology curve, when a civilization ceases use of an outdated piece of technology. In our case humanity is looking for radar because we're currently broadcasting it. Why would anyone continue using radar to communicate within a few years of quantum entangled data processing, capable instantly transmitting data instantaneously across vast distances? There might be a few living that rock star lifestyle of HAM radio operation and messenger pidgeons, but at that moment our civ would go narly completely dark to our sister civ's SETI program. All humanity's existence so far has been nothing but spark swallowed up by the darkness of time. Our use of radio a fart in the wind.

This theory is incredibly type-zero-civ-pocentric. A species capable of interstellar travel will have mastered technologies we can't even comprehend. By the time humanity is able to meaningfully reach across the stars, we'll have spread life across our entire solar system and everything we'd ever need. Unlimited energy from our sun, a lush and verdant Venus and Mars, mining colonies across the solar system producing vast quantities of any desirable element, not to mention an Earth whose biosphere is a shining jewel - perpetually locked-in at peak biodiversity.

Begs the question of what we would ever need from another civ, and furthermore what a similar civilization would ever want from a bunch of squatting troglodytes such as ourselves.