"Never seen again" is a relative phrase. I think that probably Otzi in the Tyrolean Alps and Cheddar Man in southwestern England were both probably what would be called today "Missing 411" cases. They were eventually "seen again", but it took about 11,000 years in the one case and about 5,000 years in the other to find them.
No doubt throughout history there have been many other such cases, but many of those will truly "never be seen again" because there is nothing left of them to see - it's just that those two cases had unusual preservation of the corpses that allowed them to be found after a few thousand years.
The difference in the modern day is that tehre are now a lot more people, a certain fraction of whom will vanish at some point "without a trace"...
.
No doubt throughout history there have been many other such cases, but many of those will truly "never be seen again" because there is nothing left of them to see - it's just that those two cases had unusual preservation of the corpses that allowed them to be found after a few thousand years.
The difference in the modern day is that tehre are now a lot more people, a certain fraction of whom will vanish at some point "without a trace"...
.