That said, though, why assume that a colonization wave is astronomically detectable? Being detectable at this range with our instruments is surely an indication of wasting rather an enormous amount of energy that could instead be put to use by a sufficiently advanced technology, no?

Waste heat is one thing there's not much one can do about. Even a Dyson sphere will have it. In the case of Dyson spheres there have been active attempts to find them. See here although some other work suggests that Dyson spheres are just not that likely(pdf). Most largescale engineering projects will leave a recognizable signature. In this example, systematic searches have only been done out a few hundred light years, but stellar engineering is in the more blunt forms noticeable even at an intergalactic level.

Moreover, many ship designs lead to detectable results. For example, large fusion torch drives have a known sort of signature that we've looked for and haven't found.