If humans disappeared today, and aliens visited in the future, for how long would plastic or other "obviously" non-natural polymers be detectable on the planet, using known or plausible technology. That is, how likely would it be that the aliens would say: "Clearly, the presence of these molecules indicates past technological activity"?
I'm mostly interested in accidental preservation of plastic (buried in sediment, microplastics in the ocean, etc.), but also the long-term stability if, for instance, someone intentionally tried to preserve a plastic artifact. Could it survive for thousands of years if adequately protected? Millions? Billions?
Comments imply that if isolated from biological activity, at least some plastics are sufficiently stable to survive for billions of years (I assume that requires temps below, say, 40C or so?). Is it plausible for plastic to end up in an arid region, get buried in sediment, and survive for millions of years intact while getting buried under successive layers? At what pressure would the plastics degrade and no longer be recognizable as the product of technology? I assume this would occur well before it turns into coal.
Finally, while I recognize that this is the Chemistry SE and not Biology, are all plastic species today degradable by some known biological organism (meaning, for each specie, does there exist a known bacterium that eats it)? Is there a reason that life would not eventually adapt a way to "eat" every kind of plastic/polymer we produce (like, not enough energy to recover, etc.)?
Note: I have basically a high-school level understanding of chemistry, so apologies for the ill-posed question.