Could this be Cleome?
I've found a few of these seedlings in my wildflower garden and one that made itself at home with a potted tree. I have a cleome in a pot about 10 feet away from these areas and it has already finished flowering for the season (we never got a winter this year) so I'm wondering if some seed pods got loose and made their way to my flower beds. I tried to Google pictures of cCleome seedlings and they didn't look quite like this. The leaves resemble Cleome leaves as far as shape but they are missing two leaves on each. Any ideas?