That's actually something I wonder if I could make a detection kit for. Reflection of finger grease on buttons. Makes sense that the first button you press will consistently have more grease on it than subsequent buttons. Would it be linear? Logarithmic?

If I still had any get up and go left at the moment I'd give it a go but now is not the right time. Let me know how you get on if you try. I'd go for light refraction if I was going to try.

You're welcome.