I figgered it out Chief! If Aliens are 4 dimensional beings while we are only 3 dimensional, even if they accidentally crashed or left a singularity generator on Earth, with an extra dimension of freedom they could reach through any safe or high security area and remove what they wanted with no obsevation. All we would know is that evidence was there and then it wasn't. That the chinese dropa stones are still here may be an indication that they are 3 dimensional too. That or they simply forgot about them. So whadduya think chief...chief? Where didja go chief?