I think the silurian hypothesis still holds. It's unlikely that these plastic rocks would remain in their current form for the next x million years.

Or they never used oil. Or better managed their use, with no waste of the form.

Not to say I believe a iota in such an hypothesis though.

On a long enough timescale, any trace of a precursor civilization would be taken into the earth's crust through tectonic shifts; still curious to think about though.

Closer to today would be Atlantis, if it was an island then it might still be somewhere under the sea / sediment.

Well, maybe. To name one example, the Canadian Shield, though heavily eroded, has never been subducted. You can still find exposed Precambian rock there. To name another, the Burgess Shale (also in Canada) contains fossils of some of the earliest known complex organisms.