@Ramey3
I agree with @Mike-And-Jane0 . If the garage is not part of the listing, and there is no guest access, (and you have disclosed the garage is no guest access in the Guest Access section under the Description), then technically the camera inside doesn't need to be disclosed and doesn't violate Airbnb policies.
That said, this is such a touchy topic with Airbnb. If you really want to cover ALL bases, I would put a camera on the outside of the garage as well. I would disclose the outside camera on the listing and mention there is no guest access to the garage in the Guest Access Section. This would be a further theft deterrent if guests could possibly walk around the footprint of the garage and is probably a good idea anyway even when guests aren't staying.