I was watching this video recently posted on The Angry Astronaut YouTube channel. The video discusses the Proxima Centauri signal that was in the news last month and a few other notable candidates for extraterrestrial technosignatures.
The video brings up the point that each time a possible technosignature is detected the scientific community tends to immediately try to postulate a model which would be explained naturally without aliens. Obviously not every new phenomenon discovered is aliens. For example the first radio pulsar discovered by by Jocelyn Bell Burnell.
The video points out what may be signs of artificial technosignatures such as the unusual amounts of heavy elements in Przybylski's Star. But every possible technosignature tends to be explained away as natural with models by the scientific community at large regardless of whether or not those models are based on assumptions and hypotheses that match the observations.
Is there anything that could be a technosignature that would be undeniable and have the only possible explanation be a technological extraterrestrial civilization?
I think the discovery of a black dwarf star would be the smoking gun. Currently there should be absolutely no black dwarfs in the Universe. The absence of black dwarfs is one of the proofs that the Universe is relatively young so the discovery of just a single black dwarf would be a blow to cosmology.
Consider the Kardashev scale. A Type I civilization might construct a Dyson sphere to harness the energy radiated by its parent star to become a Type II civilization. A Type II might envelop every star in its galaxy with a Dyson sphere to harness the energy of the galaxy and become a Type III civilization.
If the speed of light or another fundamental factor prevents a space-faring civilization from colonizing neighboring galaxies then the society can stagnate at its ability to use energy, go backward technologically or find ways to become more energy efficient. If the speed of light is an unreachable speed than most likely 100% effeiciency is also unobtainable.
The only realistic option to increase the amount of available energy to harness for a Type III civilization when the fundamental laws of physics are insurmountable may be to artificially increase a star's fusion rate. If a white dwarf could be manipulated to fuse its material more rapidly the star would reach a black dwarf status sooner rather than later. Once a black dwarf forms the Dyson sphere is obsolete and can be disassembled.
The question is if a black dwarf can be detectable with contemporary methods. Probably not. Black dwarfs would emit very little light in any spectrum. A black dwarf's gravitational influence on nearby objects would be a better indicator of its existence.
Maybe black dwarfs have already been detected as a small fraction of our observations of "dark matter". Even though black dwarfs emit very little light they still might used by Type III civilizations for their strong gravity. Black dwarfs could hypothetically be systematically arranged to attract and absorb space debris.
Or maybe astronomers would just hypothesize a new type of black hole if they saw that happening. It's a huge Universe so anything is possible. If scientists weren't biased in some way they wouldn't keep getting jury summons.