Worldwide there is a "Great Unconformity" where strata ~500 million years old lay directly on top of strata 1 billion years old...

The theory I heard was that the Glaciers of 'snowball Earth' essentially erased 500 million years of geologic history...

There was a science fiction book that had- as a minor plot point - the revelation that a sentient technological species existed on earth during this missing period...

It was 'The Light of Other Days' by Arthur C. Clarke and Stephen Baxter...

Perhaps I do not have a deep enough appreciation of how deep 'deep time' is- but I think that a global civilization like ours would still leave easily discernible traces even after a half billion years... Just look at the extensive mines we make, the precisely patterned geological alteration of huge swaths of land we make with our cities- those will be obvious for 100's of millions of years. The glasses and ceramics we make that are of obviously intelligent design will be far more likely to be preserved than bones.

We have even found the remains of a 2billion year old (natural) nuclear reactor and dissected it enough to reconstruct an idea of how it worked... (Don't tell Johannes)

One of the reactors released 15,000 MWth.years of energy and consumed six metric tonnes of uranium. It operated over several hundred thousand years at a low average power of 100 kWth. Sixteen reacting zones where these natural reactors occurred have been identified.

The strata laid at the bottom of the ocean during the anthropocene will bear chemical evidence of a technological civilization...

I think that we can safely assume we are the first technological species on Earth, since had there been another, we would know it already.