The Earth has been visited for eons, long before mankind walked the Earth. Your Earth is fertile and full of life, an endless source of interest to the curious. Just as travelers on long sea voyages, or trekking long distances over continents, were lost to their human families, just so these visitors on occasion lost their moorings. They died, and their companions were not able to locate their remains. Dead tissue is dead tissue, and it does not speak but rather blends into the surrounding forest or swamp, lost to those anxiously looking for a companion. Humans are aware that their technology is not perfect, and this lack of perfection is always present. The phone may be but a fingertip away and the ambulance can come at a moment's notice, but if one's portable phone is broken when one is dying away from home, then one may never be found!
Most often the Bones of Visitors are not recognized for what they are, as they are thought to be remains of an animal of some sort. Remains are also usually disturbed by scavengers, eaten and dragged or thrown about so that an intact section cannot be pieced together. Any remains that would tend to look hominoid, but not human, would frighten a human coming upon them, and these types of situations tend to result in active denial. The remains are left where they are found, and even if documented and pressed upon colleagues, denial prevents them from being taken seriously. If for no other reason, denial occurs because the researchers have difficulty realizing they are not alone, not the first or most intelligent conscious species in the Universe, not the masters of all they survey. One looks the other way, walks away, and then forgets to mention the find in their report.
Skulls of hominoid visitors, as well as intact skeletons of non-hominoid visitors, have been recovered by human researchers and documented. These discoveries never make the press, as the media find them too disturbing. There are solemn moments, hushed silences when no word is spoken. Then someone mentions that the public might be disturbed, might faint or attack the media with claims of fraud. The decision is make to wait a bit, for some time in the future when folks seem better prepared for such matters. If the bones of visitors were simply odd skeletal structures, they could be displayed as an example of an extinct animal, and at times this is just what they are taken to be. The bones that don't get displayed are hominoid skulls that obviously have more brains than mankind, big heads that humans today don't possess, much less the heads of early man. Who wants to break this news when they would have to be faced with all those egos out there being taken down a notch! Better to keep mum, at least for the time being.