Here's yet another way to search for advanced alien civs, monitor stars to detect transits of big normous artificial structures bringing Niven's "Ringworld" to mind. Different shaped structures will yield different photometric blink signatures. Remember that something like only ~3% of orbits will appear 'edge on' to us and afford this possibility. A long shot. Somehow, I don't think this method will meet Seth Shostak's 2020 or so date.

From SYSTEMIC okolo.org

On a related note, it might be worthwhile to sift through all those CoRoT lightcurves for photometric banner ads. As Luc Arnold writes in the abstract for “Transit Lightcurve Signatures of Artifical Objects” (astro-ph/0503580):

The forthcoming space missions, able to detect Earth-like planets by the transit method, will a fortiori also be able to detect the transit of artificial planet-size objects. Multiple artificial objects would produce lightcurves easily distinguishable from natural transits. If only one artificial object transits, detecting its artificial nature becomes more difficult. We discuss the case of three different objects (triangle, 2-screen, louver-like 6-screen) and show that they have a transit lightcurve distinguishable from the transit of natural planets, either spherical or oblate, although an ambiguity with the transit of a ringed planet exists in some cases. We show that transits, especially in the case of multiple artificial objects, could be used for the emission of attention-getting signals, with a sky coverage comparable to the laser pulse method. The large number of expected planets (several hundreds) to be discovered by the transit method by next space missions will allow to test these ideas.