It's far more likely to be a poorly understood natural phenomenon that we will later make sense of. It would be the millionth example of that, compared with the _first ever_ example of life not from Earth (capable of building interstellar tech!). We need very, very strong evidence to overcome this hurdle.
We know for a fact that the universe is filled with strange and interesting dynamics (not involving alien life forms) that we don't always understand at first. We don't yet know where else life may exist in the universe, if at all.
Alien life would be a natural phenomenon, ultimately; on a deep analysis any phenomenon is natural.
We make a differentiation between phenomenon wrought from cognition, but that cognition is - at least in part - a natural phenomenon too.
Is this distinction founded in dualist philosophies(mind-body and/or material-spirit)? Does it make sense to make for monistic materialists?
We might not ultimately recognise alien construction as coming from some intellect, plenty of creatures create structures on Earth (as do physical processes) with high complexity that are not considered to result from cognition.
We don't need to play this pointless semantics game every time the word "natural" is used.
There is an inarguable qualitative difference between the types of objects and materials that can be produced from undirected mechanical and chemical processes and those that can be produced through the work of intelligent, living beings.
In picking this semantic nit, you've completely ignored the actual point made by the GP: we've seen countless examples of new and strange phenomena that end up being described by undirected physical and chemical processes. We have yet to find anything that requires intelligent life to explain, like an interstellar skyscraper would.
It was more a "oh, this is a cool idea [to me, at this instant]".
Particularly I like that it sent me in to thinking of an alien life form that could, maybe like ants, construct a large scale "artefact" - like a solar sail - that we would recognise as being wrought by intelligent life but would actually be the result of some sub-intelligent life form.
I like Fritjof Capra's rendition of Gaia hypothesis (as an idea), but had never really extended the idea of self-ordering, or complex-ordering, to how we might interact with alien constructions.
>In picking this semantic nit, you've completely ignored ...//
I meant the term "natural" only as a short hand for anything that isn't alien technology. I do agree that even civilization and technology can be viewed as natural.
It's context-depednent. Unnatural doesn't need cognition-dependence, but merely higher complexity than usual -- an arthropod skeleton floating in space is unnatural, in the context of an asteroid, because asteroids are though of as "nartually" arising collisions of rocks, gravity, solar radiation, and the like, but not not the complex processes of animal life.
We know for a fact that the universe is filled with strange and interesting dynamics (not involving alien life forms) that we don't always understand at first. We don't yet know where else life may exist in the universe, if at all.