[mrangle]

Absolutely metal and plastic decompose and break down, respectively.

[inglor_cz]

Not all metal. Buried gold retrieved from ancient graves looks very "fresh".

[MichaelZuo]

After ten million years even a large gold ingot would entirely be subsumed by the surrounding matter. Assuming a 10 nanometer/year erosion.

[jiggawatts]

Also most of the noble metals would remain pristine: platinum, iridium, etc…

[userbinator]

The latter, back into oil.

[thrashh]

Sure, but on very long timescales. And eventually once you make enough of it, it gets lodged in all sorts of places, some less susceptible to decomposition, such as humans in bogs or mammoths in glaciers.

I just find it hard to believe there to be not a single trace of anything.