I pulled my last Red Norland potatoes a couple of weeks ago, pulling out the dead vines. Searching around the row for detached potatoes, I found a number of pea-sized ones that I left in place. I didn't dig the row, not to disturb the nearby fingerlings, and I replaced the mulch in case the fingerlings wanted to expand into the space.
Today I started to pull the first fingerling vines, and there under the mulch where I had left the little pea potatoes were a dozen nice-sized egg-sized tubers. These were totally loose, not attached to vines, which I had pulled and discarded. I can only assume that they were the tiny potatoes that had somehow grown on their own. How they could have done this is a mystery. I might have expected some of them to sprout, but these had just grown into larger tubers.
Strange. Nice, but strange.