Mystery plant

Yesterday Melissa had a great post. It was filled with drama, intrigue and mystery. I posted a comment that the blogworld needed more posts like that one and vowed to come up with something mysterious for my own blog. I have kept my promise:
Three of these plants popped up in my little flower bed about a week or two ago. They are fast growing, have strong purple stalks and green serrated leaves and I don't think they're weeds. It's the start of a bush of some sort, perhaps it will flower one of these days. BUT I didn't plant it and it wasn't here last year when we moved in. I have no idea where it came from. Ah, intrigue. I have tried to solve a botanical mystery such as this before: When I was growing up one summer, some very mysterious tomato plants were planted in our backyard. I know my Mom didn't do it because she doesn't garden and my brothers wouldn't have done it. It had to be my Dad but whenever he was questioned, he would smile and claim he had nothing to do with it. Even though he never admitted it, I know he did it. The fact that he still to this day will not fess up to planting those tomatoes is something I find hilarious. Whenever my Dad gets bored with life, he does something wacky, like plant a bunch of tomato plants in the yard in the middle of the night and plays Phantom of the Garden. Maybe Go thought he'd follow in my Dad's footsteps and play Phantom Gardener. Anyway, can anyone tell me what type of plant this is? It is a mystery that must be solved. I know it's no murder mystery but hey, this is as intriguing as my life gets.