Maybe because it firs put me in mind of looking in your reaerview mirror and seeing a person where there shouldn't be one. (Oh, crap. Now I'm freaking myself out more...)
Clicking on the photo reveals that it is a crop of a larger photo, one that depicts, "An August 1, 1951, photo shows a slotted peephole in the steel door of gambling room in Melrose Park that was raided by Cook County sheriff's police. (Tribune archive photo)"
Still creepy, though.