Hi diamond your correct, it is a Petunia and very pretty too,
Where is came from is easy too, it has came from a bird who fed on some petunia seeds as the plants were dying off last year, maybe not in your garden but from the neighbourhood for sure.
Next idea is, wind blew seeds from close by and one germinated.
You dropped a seed and it landed in a nice warm (heat from the concrete,
Sometimes birds or furry animals have seeds stick to their fur and these fall off nowhere near where they were picked up from.
just be glad you have it, it really is quite lovely.
Hope this helps you out a bit. just duck down when the birds are overhead LOL,
Take good care and Kind Regards. WeeNel.