I see seed leaves on the bottom left picture, spreading left and right. Maybe the pair of true leaves grew really quickly. Do you have picture of the seeds?

I thought I had a few seeds left over, but I'm darned if I can find them. From memory, the seeds were brown in colour, and similar in shape (but slightly smaller), to a hardy geranium.
Here's another photo from a different angle. There still appears to be only one set of leaves.

Do the seedlings smell of anything? They remind me of cress. The seeds germinate very quickly and smell quite pungent, they also get a gelatinous coating on them but hard to tell when planted below soil level.

suemcwirraluk: I think you're right. I found another packet of seeds, named curly cress, which looks exactly like them. Here's a photo via Google. If only all seeds would pop up that fast!