Hi Kay. Do you have a photo of yours?
Lori, the photo above is in April 2011 with lots of rain. I planted it out in the raised bed that spring and it's been there ever since spring of 2011.
And you could be very right about there being a stray seed. It would have had to come in the potting soil that the plant came in.
Here is when it appeared 9/2/2011. The photos before Sept 2011 show no signs of it being there. It must be a fast grower.
Sad thing is I never saw it again after the last photo in Oct. 2011. It was not there this spring.