If you are intending to use a video camera in a public or private location, you do not need to put a sign. This implies you can place a camera in front of your house without having to post a sign. A camera in your garden should be OK as well. The same is true for corporate security cameras. They do not require a sign if they are located in a public area or within the viewing angle of the camera.
However, if you want people to know that there is a video camera recording them, then you will need to put a sign up. This means anything written large enough to read from a distance - e.g., posters, flags, yard lights - can act as a warning sign. Smaller signs can be used if necessary but they should be placed close to where the camera is located so they can be easily seen.
Corporate security cameras often have their own signage requirements depending on the state or province you live in. For example, some jurisdictions require you to post a sign if you intend to record images with a camera, while others do not. Before putting up a security camera, make sure you understand the regulations that apply in your area by contacting the local government office. They can provide information about any requirements that may not be obvious from just reading the law.
Finally, keep in mind that not all violations are worthy of being recorded.
You may always check with a California state official to be sure, but it should be fine. If there is no signage required, then there is no requirement to remove the device either.
Cameras are considered "permanent fixtures" and so they don't require a permit from the city or county where they are located. However, if you want to put up a sign, then you should apply for a permit first. Some counties may have specific regulations about where you can place your security cameras. For example, San Francisco requires that all surveillance equipment be mounted at least 10 feet off the ground. Otherwise, the police can claim that you were trying to hide something illegal (such as a drug lab).
Generally speaking, if you have a security camera and it's not hidden, it's legal. So go ahead and post up some signs!
State regulations vary, but in general, you do not need to put a notice if you are using a video camera in a public location, such as in front of your house, or a private area that belongs to you and is not utilized by the public, such as your back yard. However, if you are recording images that may be viewed by others (such as security cameras at businesses), then you should get permission from those people before filming them.
The only time I would recommend posting a sign is if the camera is located on an exterior wall with no other way for pedestrians to access the premises. For example, if there is no door or window on that side of the building. Then anyone walking by will be able to see what's going on inside the property.
In this case, I would advise putting up a "No Trespassing" sign to make sure everyone knows they aren't allowed inside.
It's also important to note that most states have trespass laws that apply when someone without permission records you through a surveillance camera. So even if you don't post a sign, someone could still file a trespassing complaint against you with police if they feel like it was done maliciously. In this case, they would have the right to go into your home or business without having to give you prior notice.