And of course it only needs a fairly small probability of being implemented. If there's a 1% chance we will ever bother to do it, then you only need 69 similar civilisations in the entire galaxy for it to be more likely than not that one will eventually come and visit us.
I suspect at least most species would avoid colonizing potentially life-bearing systems. We would still detect them around nearby stars, though.
How would we detect them? Do we have radio telescopes sensitive enough to pick up communications from an intelligent radio source say even 20 light years away? To transmit recognizable signals across such distances would involve large amounts of energy, similar in scale to to the energy radiated by small stars, I would imagine. I am not well read on the subject, but based on what I know about the propagation of waves and losses through geometric spread and damping, I can't imagine this would be a straightforward task. Now if the alien species had the ability to cover up entire stars, or cause a significant change in their brightness, and they did this to enough stars, that would probably get noticed by astronomers eventually.