I came up with following technosignature:

Imagine a star system where there is CO2 rich planet closer than the main habitated planet (think earth as main planet and Venus as CO2 rich inner planet). At some point civilization will try to terraform the CO2 planet by venting CO2 into space, but they don't want to poison their own planet so there will be seasonal gaps in CO2 emissions just when the inner planet crosses in front of main planet.

So to find such civilizations you only need to know 2 orbital periods and monitor CO2 or other gas emissions. Any drop during the conjunction is the technosignature.