As far as I can make out the "24h" lamp is the 24 hour anti tamper alert saying there's been tampering.
Have a look at this thread for a bit of guidance. The tamper circuit is essentially a chain of normally closed switches so if any open or if any wire breaks it will register.
https://www.thesecurityinstaller.co.uk/community/topic/19690-tamper-fault/
Djddj
September 2022