I installed system about 3 years ago. Works just fine. However last couple of days been getting single beep from control panel, but no indication what's wrong. Have deleted most of the sensors to try and isolate the problem. No luck. Alarm works OK and I change batteries when appropriate. Everything seems as normal but I'm reluctant to just turn off the warning beep which sounds about once every minute. No disturbance of sensors therefore doubt any problem with the tampers. Could it be the rechargeable batteries in the control panel need replacing - no mention of this in the manual. Help.