Accidental Garden Additions

Last fall's bulb plantings included some bulbs that sneaked into their packages as illegal aliens, riding along with some ordinary tulips and Chionodoxa. I also know that squirrels didn't add these bulbs, unless the squirrels went to a garden center and came back with some lovely parrot tulips (I have never purchased parrot tulips). These showy tulips don't blend well with their surroundings, so they were snipped and put in a vase to be admired inside the house!
Parrot tulips growing among my Giant Beauty tulips:

The second non-invited garden bloom is this six-petaled white flower growing amid a clump of chionodoxa. From some internet searching, I suspect this is Ornithogalum umbellatum, the Star of Bethlehem.

I can't figure out where it came from, since neither Botanus nor Veseys have this bulb in their fall catalogs (my usual sources of bulbs). Also, we are in zone 1b and O. umbellatum is variously listed as being hardy to zone 4-9, and possibly zone 3 if heavily mulched. This plant is growing in my sloped rock garden in full sun. During the winter, the only protection it would have received would be the dead dry leaves of perennials and snow. Besides the unlikely climate, this plant only survived because I've had less time to weed the flowerbeds this year.

Are there any other cold climate gardeners out there with Star of Bethlehem in their gardens? How unusual is this anyhow?