Plot device for a detective novel

I noticed in the maths lift the other day that on an LED screen a "2" is a mirror reversal of "5." I feel sure this could be be used in a whodunnit thriller of some kind. Perhaps the murder victim took a photo moments before he died in an elevator, the picture seeming to show that he was on level 5, thus granting an alibi to someone (known to be on level 2 somehow) who would otherwise be a suspect. Let's say it was a double murder, with the other victim (not the photographer), partly visible in the shot. Near the end of the book, the strained detective (possibly suspended from the force by his no-nothing lieutenant (or if a private investigator, fired by the victim's newly wealthy heiress(which may or may not be a red herring))), suddenly discovers upon closer inspection that the wedding ring of the other victim is on the wrong hand, and thus, that the entire shot is actually of a mirror opposite to the lift door. I could weave in some subtle hints about mirrors throughout the book, so much so that an avid reader would, upon conclusion of the mystery, grasp her head in her hands and exclaim "Of course, the answer was staring me in the face all along!" But I can't write that novel now. I've just given away the ending in a blog post