You really shouldn't worry about full fledged hibernation/brumation in Southern California. I know that California is notorious for having wacky climatic conditions, but as long as your temps don't drop below 45 consistently at night...I just frankly wouldn't be that concerned that she's not hibernating. They prefer to dig down when the temps drop, but they really don't need to unless it requires getting below the frost line. If you do have a frost warning, and she's still not digging in just bring her inside.
There are two things that concern me. While your turtle doesn't really need to dig down unless there is a frost warning, if the temperatures drop much below 50 at night she will be very vulnerable to predators. A turtle's movement and even sensory capablities slow considerably at temps below 50. I have some turtles that I'm medicating right now, and I can do virtually anything to them without them reacting now that the mornings are cold. They are sitting ducks for predators. Fortunately, mine are in their "dens" for the winter. Long story short...make sure that she's protected from any predators, even neighborhood cats and field mice when the temps are below 50.
Secondly, I'm just concerened that she sounds terribly insecure and even frightened. The fact that she's eating is great, but something is obviously wrong. It could be many factors;
-she's still not accustomed to her surroundings
-she's ill
-a male is hounding her
-too much handling
-the presence of a dog or curious cat
-box turtles are not considered social animals and may actually have to adapt to being around other turtles.
-and on and on
Obviously, this is a behavioral problem. The trouble is, behavioral problems can have many causes. You're going to have to use your best judgement with this one. Your response can be as simple as "wait and see", or you can start with a trip to the vet to rule out illness or parasite infestation. You may need to separate her from the others until she's more comfortable.
The fact that she's eating is a BIG plus! That's usually the first thing to go when they're stressed out.
One rule of thumb...unless your turtle is very accustomed to captivity and you, if you put a turtle in something it will want to get out. They have to want to be in a hide box, and they usually have to go in themselves. Even if she doesn't run into the shelter, provide her both a hide box and a nice pile of leaves or other yard refuse in her corner.
I'm sorry about the long reply. I didn't really want to reply to your post until I had the time to write this all out. I hope it helps. Basically, I wouldn't be concerned about hibernation. Focus on making her feel safe and secure. One of the best ways may be just to leave her alone unless the situation really requires your intervention. It's always a good idea to quarantine animals for several months before adding them to your existing colony. Good luck, please keep us updated on her status. It's really a good thing that you take the time to observe your turtles' behavior.