I think it might be Mentha x piperita, bergamot mint, which has naturalized in Southern California.

Here is a link that might be useful: Mentha x piperita

Hmmm. The Mentha x piperita looks like the leaves have jagged edges. These leaves have a smooth edge with no tooth. They almost have a mint chocolate aroma. Just delicious!

The previous owner has connections to a local nursery in town. I wonder if she planted it. She planted some amazing things.

It's growing in the wrong place, like mint often does. I have a feeling a previous owner planted it, regretted it, ripped it out, and with some rain and irrigation, a remaining root is now growing.

We'll be potting it up today.

Suzi