My Money would be on hard drive, it sounds like you don’t know that kit so probably don’t know if all the system parameters are set up correctly
certainly worth getting a professional in to give it a once over
One solution would be, fit a mobile 4g router (with data sim in it) & connect this directly to his Cctv, this would give him remote viewing.
granted would cost him a few extra quid a month topping up pay as you go data but it would work
Ive used this method a few times to remote view empty properties that have no internet connection
As per previous comment, adjust the spring & ensure the battery is at a healthy level
some sit on a spacer, so make sure that’s not causing any issues Or that the spring is not bent (has that before)
if you want to remove from the zone you need the engineer code & it’s done on the program zones (first option when in engineers mode)
I know it may seem like an expense, but at the end of the day if regularly maintained & kept updated / in good working then it will give you peace of mind...
With respect some people just see an alarm as fit & forget
be mindful to replace them with the correct sized quick blow fuses (if you don't intend to get an installer in & DIY it)
The fuses are sized to prevent damage or possible fire!
You need to check the zine you trigger is programmed as a 12hr (ie instant alarm on zone)
failing that press & hold 1 & 3 on the remote keypad to activate the panic, this will give you a full alarm condition
As above possible damage charging circuit.
If you don't know anything about these alarms with the greatest amount of respect should you (if you are a company) be touching them?
You may rectify the fault but there may be other issues within the system - you could give your customer a false sense of security thinking all is ok - not being funny when I say that just an observation
AC voltage in seems about right & you say you have disconnected the battery
Have you power cycled it? (may be worth powering it off for a short duration then powering it back up)
If all that fails then I suspect its a faulty PCB.
Assuming its a coax based system and not CAT 5, can you review the cabling routes on the cameras with issues?
Do they run through any switch rroms?
Are the cables routed next to any LV / HV cables?