The alarm may be operated by a simple hidden two-way switch - such as a light switch. If you want more security - you can use a key switch - or one of a number of code operated Keypad Switches.
Before you set the alarm - make sure that the building is secure - that ALL of the Green LEDs are lighting - and that the Yellow LED is off. If the Yellow LED is lighting - there's a fault in one of the zones - and THE ALARM WILL NOT SET.
Depending on the setting of R9 - when you move Sw1 to the "set" position - you have up to about a minute to leave the building. When you return and open the door - the Buzzer will sound. Depending on the setting of R8 - you have up to about a minute to switch the alarm off. If you fail to do so - the Siren will sound.
Depending on the setting of R14 - the Siren will sound for up to about 20-minutes. Then it will switch off - and remain off. Of course - you can stop the noise at any time by moving Sw1 to the "off" position.
When you return - if the Buzzer does not sound and the Yellow LED is lighting - then there's been an activation while you were away.