MindyT
Aren't you lucky, having such a beautiful plant arriving in your garden and free to boot
Yes it is very possible for the seeds to carried in the wind, dropped by birds or even carried by insects. The seeds of Hibiscus or as you know it The Rose of Sharon take very easily.
There are quite a few varieties of Hibiscus, three of which I think ,are already mentioned on this forum under the headings 'Hibiscus as a Tree' and 'Please Help me out', both of which have quite a lot of plant information and pictures.
Am not sure what your climate is like, but have to assume that you live somewhere where Hibiscus is quite a commonly grown plant, as a seed sprouted in your garden. in which case you would find that they are very easy to grow, as they almost take care of themselves, with only additional care needed depending on the variety of your Hibiscus or your climate.
Do you have a photo of your Hibiscus with its light purple flower that you could post, as this would help identify which variety you have and therefore make it easier for us to help with any extra care instructions your plant may need.