I think your comparison of the black Kettle orbiting Uranau full of gold is not valid. We know that very different humanoid beings existed in the past, we also know that atleast one type has survived to this day. From these facts, it shouldn't be considered extraordinary at all that another could do the same. The argument that they've gone undetected doesn't wash either, They've been seen and reported for hundreds of years. The thing that is lacking is full recognition through irrefutable documentation and a better understanding why that is. It could be that we can't tell their bones from ours very well, or that we tend to recognize that they just don't belong in zoos or living within our society. If you couple that with the prospect that they don't want to be scientically observed or studied, then it would go a long ways towards explaining the sparsity of recognised evidence or proof.