I'd be interested in the amount of truth behind this.

It it established Google keeps "shadow" behavioral data that isn't accessible to users?

I'm sure they have some in other systems to some degree (i.e logging).

I just did a takeout for my location history (which I keep off) and it was empty.

Do you have any evidence besides "well it's technically possible, and they might be violating GDPR and lying in their product documentation"?