out of place plant

About a week ago, I was pulling weeds in my vegetable garden and noticed a little green plant that was out of place, but looked a little too familiar to be a weed. So I decided to let it grow for a little while and see what it was. Then, a few days ago, I looked out the window at the garden and noticed a bright speck of pink at the back. I thought it was probably a piece of trash or something that had blown in there. But when I went out to look, it turned out that the out of place plant was a petunia and the pink was the first flower. I have no idea how a petunia sprouted in my garden, I've never even planted any flowers in the back yard at all. Since it's growing amongst the green beans that are dying from the heat, I think I'm going to leave it instead of trying to transfer it to someplace else.