Interesting point that at some point your observing apparatus gets good enough that you can 'see' the structures built by sufficiently advanced civilizations (sure they cloak their ships in orbit but you can see how they make their home world comfy!)
At one of the SETI seminars there was a discussion about when would be the "right" time to alert a newly discovered intelligent species that they aren't alone in the universe. There was a lot of back and forth about indigenous tribes in the Amazon, some of who learned of other tribes by the arrival of missionaries, some by loggers, and some who were out walkabout and came upon the strangers. How you meet outsiders has a different impact on how it affects you.
So if you were aliens and you didn't want to 'alarm' or 'damage' humans, what would you use as a signal that it was probably a good time to say "Hello" ? I've always felt that once you could detect they were having conversations on other planets you would now "know" we weren't alone and someone could appear in orbit and say Hi. Others felt it would only be safe if humans felt reasonably confident in their own ability to meet them at their level (so perhaps at least colonies on other solar system bodies). One person at our table was firmly in the only when it is unavoidable, which is to say they are about to send a probe to an inhabited planet or come across a construction like a station that is not easily concealed or moved.