I have a small patch of russets that have turned perennial here.
They must have put some tubers way down into the subsoil, and keep coming back from those. It is also on the southwest (shaded) side of my garden which is last to lose its snow, so that may help to keep the soil from freezing. We don't really have cold winters here compared to continental places, but spring tends to be nasty with lots of frosts after snow cover is gone.
I've also seen at the farm that stray potatoes can still be a problem the following year, when you planted something else and up they come as a weed. I will have to comb through my potato bed carefully before I plant garlic there this fall.
