This is assuming the Mansion does exist.
I came up with this yesterday while watching Supernatural. (Don't ask) An EMF Meter can detect radiation so is it possible to be able to locate the Mansion with one? I mean, that place must be loaded with radiation. I know I know, "That is the stupidest thing I've ever heard" but let's face it, no one has ever tried. People look for it with nothing but their eyes to help them locate it. Now, Slenderman wouldn't just build a Mansion in plain sight, would he? No, it must be invisible to us! But radiation would linger in the air around it!!
Think about it....
-L.V