This app was mentioned in 4 comments, with an average of 2.25 upvotes
I know another poster suggested security cameras (but it wasn't on your property). If you have an Android phone...I use an app like this one. I have the first generation...but basically it will record video/audio on your phone without turning on your 'camera'.
So the benefit is, you can start recording fast, it won't make a recording noise, it frees up your phone to do other things (like call the police) all while you are still recording.
please check with your state laws before recording someone without their consent.