I am reminded of reading about the way the eye and the brain work together. That the eye doesn't see what we think we see, because of the structure of the retina, and the brain fills in the gaps, using information from the way the eye moves in little jumps - saccades, I think, and it's general knowledge of the surroundings. Given that, it could be easy for people to miss something. I think it's quite common to miss something in full view. Sometimes because the mental image doesn't match the way the object is presenting itself to the viewer - which could explain the missed mace jar. But not the glasses case, though.