It's not impossible to have a seedling if the flower was fertilized and the resulting pod left to ripen and split.

More likely the rhizome grew and traveled to new soil. Here's a picture of one that likes to stretch.

These are dwarf irises, like the ones here:
http://www.edenbrothers.com/store/iris_bulbs_reticulata.html

So they are bulbs, not rhizomes. Unless the bulbs can also form rhizomes? I dug up one of the new isolated plants, and it was round like a bulb, not that elongated knobby appearance like larger irises.

It appears these are I. Reticulata iris, They bloom with the crocus and sometimes when still covered with snow. The leaves are reedy and very thin. They are a bulbous variety and can be quite scented. I have not heard them called "dwarf", a common term for MDB's and SDB's. Suppliers can get carried away with their descripions.