Look up Technetium stars.
Technetium is a very short-lived radio-isotope. It doesn't occur naturally in nature, the universe is too old for any to have survived the big bang. When it was found in some stellar spectra, it was assumed it could only have gotten there from seeding by an alien civilization after being deliberately manufactured by them for that purpose.
Alas, it was soon noticed all the Technetium stars were of the same peculiar type, and it was more reasonable to assume the Technetium was not an artifact, but a result of some rare but natural nucleogenesis process in that type of star.