[angry_octet]

Makes me wonder if you could achieve a similar effect by spraying some residue over the keypad before the victim uses it, then looking at it after PIN entry. For example, a fluorescing dust. As well a special fingerprinting powders (e.g. https://optimumtechnology.com.au/latent-fingerprint-powders/) you can get stuff from art supplies stores: https://www.glowpaint.com.au/blue-uv-black-light-powder/ .

There is also thermochromic ink, e.g. a grey ink that changes to colourless at 15C. http://www.smarol.com/Ultraviolet-Fluorescent-Powder.html

At this point, I don't think it is viable to pretend that long lifetime secrets, like your bank PIN, are safe if entered into hundreds of different keypads in insecure settings.