I saw your post but just didn't have a spare moment to reply yet.
The odds of getting one single variegated seedling and nothing else around it are extremely slim. There are much better odds that it is just a tiny bit of Linda Sue that got broken off by a vole, mole, chipmunk, squirrel, rake or weeding tool. It only takes a tiny piece of crown material, say from a broken root or sliver of an eye to create a seedling sized plant like this. I've seen it before here and after digging it up could see the little bit that it came from at the base, sort of like getting a TC plant.
Hope that helps a little