I'm not sure what "fully technological" means, but if they had made anything at scale out of durable materials like stone or ceramic we would most likely have seen some indication.
What we do probably underestimate is how advanced ancient hominids were. They mostly worked with materials that decay like wood, plants, and animal skin. But we're slowly learning that they were more advanced than we used to believe.
Any former civilization that is any sizable fraction of ours with same level of tech would have been spotted long ago. Well unless Atlantis is real somehow sunk 3 miles down
What we do probably underestimate is how advanced ancient hominids were. They mostly worked with materials that decay like wood, plants, and animal skin. But we're slowly learning that they were more advanced than we used to believe.