You are most likely correct in it being Ribes sp., probably from a seed dropped by a bird. I probably didn't notice it at first because the foliage is very similar to the columbines growing in that spot. Haven't found any pictures that show foliage quite like this one; they mostly appear to be more rigid and less deeply lobed. These leaves are very soft, the texture is similar to those columbines it was growing with. And there are no thorns. If it survives my rude extraction (looks reasonably ok this morning) and goes on to bloom, maybe we can figure out which one it is.
Thanks for steering me in this direction; I think it's the right one.
