Probably seeded by birds?
I've got some weird additions due to the berry/seed eating birds; a sort of bushy clump with little daisies (weed? camomile?) , hebe, hypericum, pyracantha, cotoneaster, mahonia.....all round the edge where they sit on the fence and poo.
And all the flowers attract bees as well as producing berries/seeds for the birds!
Ain't nature wonderful......