looks like tradescantia spiderwort or some form of tradescantia.

It's Euphorbia lathyrus, common name Caper Spurge, considered a weed, usually from seed dropped by birds... wear gloves if you pull it out, the sap can be a skin irritant

Goodness, this plant is coming up a lot on questions this week! :) I've never seen this Euphorbia here and thought it rather elegant, but am told its a spreading weed in some places.

They will have germinated a while back, but most people only notice them when they get tall or actually to the 'flowering' stage. And yes, where there's one, there'll be several the following year if left.

I've got one in the ground that the birds left me LOL, I like the flowers it has, well worth keeping :o))

I like them too LadyEssex! And.. if it was left by the birds that means they like it too.