Okay, so the title is a bit off. They’re hunting for partial Dyson spheres using infrared and optical.

I was confused on how they would detect something completely blocking a sun from millions of light-years away.

Even a Dyson sphere, which is technically unlikely anyway, would be possible to spot. You would look for something very bright in the infrared spectrum with almost no light in the visible spectrum. It would also be larger than a normal star of the same energy, but that would be hard to tell given all the other issues.

A partial swarm is easier because it will have variability towards more infrared and then back to a more normal spectrum.

And, of course, all this is speculation until we find a candidate and determine it doesn’t have a natural source for that behavior.

Why would there necessarily be strong infrared emissions? Since a Dyson Sphere is meant to harvest all energy produced by a star, any leakage would be unnecessary inefficiency, wouldn’t it?

Thermodynamics says that energy can’t be destroyed (mass-energy, but generally that won’t matter). So after the work of running your stellar civilization is done, you will radiate out waste heat. There is no real way around this without breaking thermodynamics or having a handy black hole to dump all your waste heat into. Therefore, the energy of the star will still be released, but it will be released as infrared.

If you’re using the Dyson sphere purely as a power plant and e.g. charge batteries, the thermal radiation will be distributed over the whole area covered by the civilization.

A solar panel, or any other power generator we use, doesn’t radiate away all the generated energy either. It’s radiated from the point of use.

But you would still be radiating heat from that star system unless you’re proposing wireless energy transfer over Interstellar distances. So the entire system would still give off an unusually high infrared signature.