Veritas Excel Alarm?

I have a Texecom Excel Alarm in a flat. It is a basic set up with 4 room sensors and 1 exit door contact and was installed professionally 6 years ago. It has the LCD Key Pad by the exit door and the Control panel is by the electrical distribution box.

I have just noticed that when I set the Alarm and close the final exit door the Control Panel continues to emit the exit tone and does not stop until I re-enter the flat and enter the Access Code.

There are no error messages on the LCD panel.

I assume the Alarm circuit is not setting if the exit tone continues to sound?

Are there any tests or checks I can carry out to identify what might be causing the problem? I have the Master User Guide but it doesn't seem to include any information about fault finding.

Many thanks to anyone who suggests a solution.