I second some of the beforesaid, especially friction causing calluses and sometimes unconscious movements causing that friction. One question that I would ask, though: Did the wart disappear before or after you went shoeless? I am asking because plantar warts can be pushed into the skin to a point, where they can be completely overgrown by callus. So maybe your wart is still there, but "underground", leading to more friction in the surrounding skin. Regarding treatment, I would recommend urea cream, at least 10%, to soften the callus. Often, it rubs off on its own after that.