Million bells aka Calibrachoa is a tender perennial and will return in zone 9 and warmer.* This lady may have a warm spot in her garden where they return.* Most don't produce lots of seed.* In your zone they are considered annuals.* Here's some interesting info about them.
There is a perennial petunia with smaller flowers then the annuals you see at garden centers that should survive in zone 8 called Petunia integrifolia.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.