There is a trade in human remains. Many of these will be medical specimens, from cadavers of people who donated their remains. There are, of course, remains where the provenance is uncertain, or downright dodgy. Some of these will be ancient remains, or remains uncovered by historical burials being disturbed. But there are always a percentage where the chances are the remains belong to a missing or murdered person whose remains were just stumbled across, say, in the woods. A person who is not a forensic anthropologist could easily mistake skeletal remains as being 'ancient' when they could only be months to years old. They take a skull they find home, thinking it's spooky and funny, or a scientific curiosity, not realising it belongs to a murder victim from only two years ago.
It's the reason ANY suspected human remains should immediately be reported to police and disturbed as little as possible.
(And even ancient remains, it should go without saying, should be treated with the same respect as a modern Doe. Over in the US, I imagine there will be tribal organisations who take custodianship of ancient remains, out here in Australia, we have similar processes with our indigenous peoples. Gone are the days when a person can just put a skull of unknown provenance on their mantel and tell people that it's fine because it's just that of an indigenous person. Thank goodness.)
MOO