It wouldn't set the panel sounder off but will still sound the bell as soon as you lift the cover unless the engineers hold off as been wired.
Sounds like the tamper switch as been wired out or jammed/broken, (if it still works normally with bell test)
It's more than likely just a faulty tamper switch, try pulling it away from the wall it should be on a pivot making sure it still can move. Also check that the wire from the switch going the bell pcb hasn't been cut and twisted together.
You will just need a new internal module they just clip in/out with one small screw.
That's if you are really that bothered about a tamper on the bell box, how high is it? do you need ladders to get to it?
What are the real chances of someone turning up with ladders knowing that the tramper doesn't work opening it up and disconnecting the siren?
It's more than likely just a faulty tamper switch, try pulling it away from the wall it should be on a pivot making sure it still can move. Also check that the wire from the switch going the bell pcb hasn't been cut and twisted together.
Its probably jammed on the back pin on the rear plate, when you clip the internal module into the back plate you need to lift the tamper switch up when you push the module downwards.
Easiest way is just unplug the sounder on the bell box itself the strobe will still flash.
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