Accenta 8 alarm bleeping when sensor activated though not set - help!?

HI there, WE have an accenta 8 alarm which is not set at the moment because we are in. However every time we move and a sensor lights up a bleep sounds the same as the bleep that sounds each time the door to our office is opened. Also I've just tried to set the alarm to see if setting and unsetting stops it but it just wouldn't set - it just carried on bleeping and bleeping. Could anyone help please? Many thanks.