Of course we haven't discovered everything. But it seems very likely that if there were a civilization's worth of slag from 500m years ago, we would have discovered it.
A more interesting question is the civilizational technology of domestication, especially since that is short-term technology coupled with long-term evolution. It does seem possible (but purely fantastical) that Earth had talking dinosaurs with sophisticated ideologies and mythologies, feeding dumber dinosaurs and curating the growth of nutritious plants, the descendants of which all died 65m years ago.
What really happened is that some uranium got concentrated, and then it did was uranium does.
Similarly, if you want to make a very, very basic atomic bomb, you can do it by picking up two subcritical bits of uranium of the right isotopes and slamming it together in your hands. Basically the demon core but even more pure. The yield will be pathetic by military standards but you won't be around to care.
Fission isn't a process that requires intelligence, and fission causing cascading fission doesn't either.
"A more interesting question is the civilizational technology of domestication, especially since that is short-term technology coupled with long-term evolution. It does seem possible (but purely fantastical) that Earth had talking dinosaurs with sophisticated ideologies and mythologies, feeding dumber dinosaurs and curating the growth of nutritious plants, the descendants of which all died 65m years ago."
Yes, that's why I'm careful to qualify it with "technological", which is by implication the whole thing we do with metals and minerals and oil and all that stuff. A past intelligent society that lived like hippies and had all sorts of fascinating philosophical debates until they died out with no major footprint can't be completely excluded. But I am fairly confident they never went to space. Well... you know... real space, like the kind of orbit the ISS is in. What they may have gotten up to on the influence of drugs I have no comment about.
Part of the reason I couldn't exclude it is that it has basically happened. If hypothetically an asteroid blew away humanity two or three thousand years ago, that's basically what would have been left behind. They did mining then, but at a scale easy to believe could be erased by time. Quite a bit of that time period is already gone as it is, let alone across millions of years. It's really that modern industrial mining that we do today, the cumulative effect of many decades of that mining, and barring disaster, the cumulative effect of yet more decades/centuries of mining that would survive for millions of years.
Yes, that's why I'm careful to qualify it with "technological", which is by implication the whole thing we do with metals and minerals and oil and all that stuff.
In particular, there don't seem to have been any dinosaurs that had anything like primate hands, so they would have never built real tools or weapons, or manipulated fires. That doesn't close off the possibility of an "advanced civilization" unless you mean "uses tools as fancy as ours." We don't actually know what 200,000 years of natural domestication would look like.
Why is it obvious that none of them can survive? Ordinary bones can become rocks (fossils) that are preserved. If mining of iron ore is visible, things would have been made out of iron -- wouldn't a large structure made out of iron leave some trace? Or even a small one? Why are there dinosaur fossils galore, but not so much as a pre-human fidget spinner to be found?
But one counterargument that can be made is that fossilization requires very specific circumstances, and the legacy of the dinosaurs is from millions of years of opportunities. A blip of a 500 year civilization might not leave much behind.
No, I wouldn't expect a big pile of iron to leave anything interesting behind. It would rust, and rust has no structurally integrity and probably wouldn't resist the general forces of time very well. On geological timeframes rust is soluable in water.
However, I couldn't tell you that about every last industrial artifact we have. Could the Hoover Dam still indicate some sort of obvious construction activity a few million years hence? Could some of the concrete end up preserved and some future civilization realize that it is too precisely formulated for an engineering purpose to be constructed? Could some titanium structure survive for extended periods of time even though everything else in the plane or whatever else is long gone? Could New Orleans end up sinking into muck and leave some sort of fossilization behind of things that are still clearly artifacts millions of years later? It's hard to prove that every last thing we create has absolutely no chance of being recognizable as an artifact even in millions of years.
Especially if we leave Earth and start including the moon. Just a few things here or there could be wiped out with an unlucky asteroid strike, but if something was really systematically up there for long periods of time doing intelligent things, even hundreds of millions of years ago, could still leave an obviously-intelligent footprint.