One really-cool way to solve that problem is to embed a 7-segment LED under each keycap. You walk up to the keypad and the 0-9 digits appear in random order. No one can shoulder-surf, look for wear or IR emission from the buttons, or train on the click sounds.

Dell had those on every lab door in the building back in the early 90s. You felt like 007 every time you punched in your access code. I've never seen them anywhere since.

Maybe. They were necessarily very cagey about it back then, but I might have some documentation kicking about in storage. I tended to keep copies of every service manual I could get my hands on back then.