Since you live in its natural range, I think it would be a stretch to say that it is a captive animal that escaped. I can see that someone may have illegally taken it and it escaped them. Or it very well may have wandered into your neighborhood on its own. Tortoises can really move sometimes.
Either way, it is really illegal to have it. If you are unsure where to release it, others here may be able to help. I would consider calling fish, game and wildlife, or what ever California calls it. Where I live, Dept of Environmental Control (DEC) would not bust you for something like this. Instead, they would take the animal and know where a proper place to release it would be. However, I'd hate to say this and see you get fined or something. Talk to others in California and see what the norm is.
Dave