Looks like oxalis to me (Oxalis pes-caprae) variously known a Bermuda butter cup and dozens of other names. A seed may have got in with the ones you bought or by other means - the pods 'fire' seeds quite some distance!
Jimbolena, your garden looks very interesting. I like what you have done there
Thanks, it's been 5 years in the making, there were a couple of perrenials and some ground cover plants, no steps and grass at the top terrace. I quickly covered the grass with bark because there's no access to the garden other than through the house so having to haul a lawn mower through every week was upsetting my wife and me.
I love the wee pond I built this year, it's developed into a delight. The algae issue has righted itself, I'm not 100% sure how, I was told that some little algae eating critters just arrive and munch away, the lilies help aswell but anyway it seems to have calmed down, I was cleaning it every two weeks which was a mission.
Anyway check out my algae cleaner, free of charge pot cleaner....
It is one of the few weed seeds which will go through the heat produced in Council compost making operations. So you think you are being conservation minded by using non-peat compost and you get this nuisance (being polite here) coming up in your newly potted plants.