Oh, you both raise interesting questions for me. Let me see if I can set this up a little better for you.
Humans find and settle on a planet with Earth like conditions, size, distance from the sun, land to water ratio, atmosphere, etc. They are surprised to NOT find any sentient life. Many years of exploring, harvesting and building go by and they slowly see signs that some sort of higher life form must have been there. There are no buildings, no roadways, etc, but like humans they have left permanent marks on the planet. I've assumed holes in mountains where minerals were mined. I'm looking for changes to a planet that it's nature won't destroy over thousands of years. I'm assuming that the humans would eventually dig up plastics. Since, as you pointed out, they are not found in nature, they should be evidence of a lost race of people. I'm assuming that even with time and pressure the basic chemical structure would still be there.
