Looks like a bulb-type Oxalis.

If it came from a store, it would have probably been labeled "shamrock".

no store, just growing as a volunteer.

In that case it might be the dreaded Oxalis pes-caprae. Do not be tempted to put it in your garden.

Its hard to tell scale, but could it be your native Oxalis oregana?

I don’t know that one but images seem to mostly have a pale stripe on the mid vein and a more fragile looking flower stem. .... and, of course, Sod’s law says it‘s more likely to be a common invasive. 😉

Knowing the scale would be really helpful.

Floral, I agree about Sod’s law, but it still could be an option given how common of a plant it is on the west coast. You must have seen it when you visited the CA coast a few years ago. It carpets the Redwood forest floor. Here’s a picture of one growing in my backyard.

Ah, that’s more like it. Is it ephemeral? Would it have been blooming in summer?

Its not ephemeral and it lasts all the way through fall during the mushroom season. The flowers are inconsequential compared to the vast carpets that the plants form, so you wouldn't have really noticed flowers. I dont have any pictures of the fields of them on my phone, for some reason, so here’s one from the web: