answer:Sounds like a metal splinter. I expect that a wood splinter would have degraded or even rotted out by now, but a metal splinter driven straight into the skin can hold on for awhile. It can also be difficult to see, especially in a location such as you describe where you may not be able to get a good look at it. (I know that I haven’t been able to get a ‘good’ look at the bottoms of my feet in a long time.) Have someone else look, with oblique lighting and magnification – and an excellent set of tweezers.