People have been looting Graves and other historical sites since forever. So much priceless history lost. What might be out there in private selfish collections today? Stuff forgotten since the original owner thief died and kids unaware of significance or even its existence?
The int'l agreements on protection of antiquities (not NAGPRA, which was and is an abomination) mostly grandfather in material that was collected prior to some certain date. The provenance and context for such material has long been lost. Museums with non-royal Egyptian mummies have had to do a lot of work to try to figure out what they've got, and it's particularly difficult if, as is often the case, past curators unwrapped the remains. The museum in Grand Rapids has a tiny collection of Egyptian stuff, including two forensic reconstructions made from CAT scans of two mummified heads in the collection. The male head was just a head, sold to a local guy on vacation (over a century ago), by some Egyptian looter who followed him around all day demanding far too much money for it.