I've had the occasional cilantro seedling pop up here, so you should be fine in your zone, providing critters leave enough on the ground for you(I've found that mice are fond of coriander seed). Maybe collect and save some of the seed just in case?

Ditto on last year's seeds sprouting up but you'll have better luck if you collect seeds and sow them next spring.

FataMorgana

My daughter considers it nearly a weed in her zone 5 garden as it returns thickly each spring, and I have it in mine (20 miles north of her) as well. Her planting site is along the south side of a gray brick wall so it's maybe zone 6 micro-climate, but mine is not protected.

I think you could safely assume that, unless you have a thick mulch on your garden, or till the soil too deeply or just as seeds are getting ready to germinate, you will have some nice early "volunteer" cilantro!