The Continuing Story of the Balcony

I had a peek out on the balcony just now (it’s unseasonable warm, fortunately), and this thing was waiting for me:

What on Earth? I did not plant those flowers, so once again something has started growing spontaneously.

Or perhaps there’s a guerrilla balcony plant planter gang terrorising arid balconies?

What is this thing, anyway? Those are pretty flowers.