Many of the early "acheologists" (they were more like tomb raiders, invading egyptian tombs, stealing the contents, and shipping to Europe) died of misterious diseases, or got mad in some way, it is where the "cursed pyramid" trope came from.
Recently it was found out that the culprit for these, was mostly fungus, the fungus locked inside those places over thousands of years, would slowly fill the air with toxic stuff, that then would poison anyone breaking in, with effects ranging from poison-related death/disease, to poison-related brain damage (thus, madness).
Also, losing technology and weapons is not new, India Oral tradition has tales that scientists estimate they were already "mythical" 6000 years ago, and they talk about "giant metal arrows that create pillars of fire and light", "air chariots that shoot other air chariots with light" and "air chariots that fire arrows that follow heat"
Or Atlantis, that was already mythical in ancient Greece (what we know of Atlantis, is about ancient greeks writing about the Atlantis myth... so to them, Atlantis was already a mythical thing).
And of course, people would always stumble into random crap left behind by earlier generations.
For example recently something puzzled many historians, when in germany archeologists found a bronze-age battlefield with traces of having people from all countries in Europe (something kinda bizarre, since until then bronze-age was considered mostly isolationist), with signs of a large scale battle (bizarre because historians thought the population to be too small then, for that happen), and lots of signs of civilisation (advanced clothing and weaponry, including signs of that the soldiers involved were some kind of special forces, having expensive clothes and weapons, multiple scars on their bones, and yet were very strong and healthy).
Or another interesting one, here on HN someone shared recently about the people found dead on tar pits are frequently murdered, not just died there stuck, and research matched ancient roman legends (To the romans, it was a legend already), about how they heard that ancient countries threw prostitutes, adulterers, enemy nobility, and other undesired people into the tar, instead of giving them normal execution and burial.