Westinghouse Security System chirping

I have a very old security system - about 10 years old - the panel says westinghouse. I don't have it hooked up with a security company. It is connected to my doors so every time the doors open, it beeps. Yesterday, it started chirping every minute or so. It did this sometime about 2 months ago, and I replaced the battery in the control box (12V rechargeable). Now it's chirping again. I took the battery back to have it checked, but it's perfect. I can't get the chirping to stop! Please help!!