No one knows quite how long a space must remain concealed for it to acquire this particular property, nor if there are any specific conditions it must meet. But it is a far more common occurrence than you might think.
In newer buildings, when this happens, the residents often report feeling cold when passing by, even in attics during the hottest of summers. Whenever contemplating taking a quick peek to see if there is anything actually there, an unnatural dread seizes them, and they leave the room quickly, if not quite running. Once left behind, the feeling passes, and it is quickly forgotten, or laughed off.
What actually happens in these forgotten sanctuaries of the dark? It is impossible to tell. For while many such corners have been exposed to reveal absolutely nothing, some brave souls have lost their sanity through nothing more than an ill-timed glance. The safest thing to do when encountered with such a phenomenon; close your eyes, rip away the area's covering in a single motion, then keep a tight hold on what you've pulled away. No matter what you hear or feel, do not get up, do not look around, and do not try to cover your ears. You might be one of the lucky ones.