What remains from ancient advanced, technological societies is mostly stone ruins, all over the globe. After all, the survivors of cataclysms and their descendants have used and re-used whatever they could recover from these ruins. After 12,000 years what is left are the cyclopic, multi-ton building blocks too heavy to move and re-use. Any metal, not rusted away, would have been smelted to make new tools. However the ancient were able to mine, refine, and purify metals that required high temperatures to melt, such as aluminum. The survivors, having lost their technology, were not able to re-melt these kinds of metals. These metals were simply re-used, or kept as a curiosity. I think that modern-day finds of such metals, in archaeological digs or accidental finds, were just whisked away in the basement of museums, or to end up in private collections, because like so many other ancient artifacts, if it doesn’t fit the established theories then it doesn’t exist.
Once in a while such a find does get publicity, if it falls in the right hands. I have gathered the few instances when this happened, and you can read about it in my article Ancient Artifacts Made from Highly Unusual Metals .