Optima 2 Plus - stopping the tamper beep?

I have a Optima 2 Plus system, and I need to temporarily disconnect one of the sensors (in order to allow the room to be replastered).

I have followed the procedure to Omit the zone concerned. However when I went to unscrew the case on the sensor, the tamper light came on and the keypad beeped continously (althought the main external bell did not come on). So does anyone know what combination of keys I need press to stop the bleeping? (I guess that I am looking for some kind of 'Tamper disablement' but not sure where to look.)

Any advice greatfully received!