Where are you going to look for something you have no idea what it looks like and how do you know when you've seen it? Doesn't sound very scientific to me.
>our capacity to capture and "upgrade" slow photons is already improving
Due to annoyingly inescapable thermodynamics, that would require putting all the energy back that you extracted with the Dyson sphere.How about "Sisyphus Sphere?" :)
>a technological civilisation that considers a star as a convienient fusion energy source is wildly unlikly to have missed any possible way to control, direct, store and transmit energy or matter that physics will allow for
Yeah that makes sense.It's still possible they tried all those, and leeching off a convenient pre-existing natural fusor (with no build or repair expenses, for that subsystem at least) is simply the cheapest option.
I can already hear the obvious reply: "But who cares!! If they're so advanced who cares about money??"
Even (especially?) for an advanced technological civilization, you still want to choose the cheaper of two options, because some resource will always be limiting. You don't get smart by making dumb decisions. ;)
TL;DR No Free Lunch defeats Indistinguishable From Magic