Propagation: Another Unknown Seedling . . . HELP!, 1 by Xonwick

Xonwick wrote:
This appeared in one of my planters and I have no idea what it is (or will be when it grows up!)

It doesn't resemble any other plant I started from seed, the soil was brand new (bagged) commercial top soil and it's on the balcony of my 3rd floor apartment so I can't imagine where the seed came from. The only thing I can think of is that the seed came from my bird feeder but even that was filled using a 'song bird' seed mixture.

Thanks!!