Totally flat ….external bell rings for bell
Cut off time then stops ( depending on external sounder ) power back on panel will start going off , enter code to stop , reset all should be same as before …but it has been know for the Veritas to factory default !
What makes you think the outside sounder would go off after removing panel lid ? Only a down power or bell fuse replacement could set off external bell if it has a battery fitted in good condition
Only time I’ve seen this happen on one of these alarms is when …a screw was put into the plasterboard without a plug …it came loose and jammed the smoke head from sliding out …
Disconnect garage keypad , there should be a 680 ohm resistor at the last keypad that needs moving if it’s fitted at the garage keypad ( across 12v and D1)
Check fuses in panel probably 2 of
F1 1A quick blow
F2 250ma quick blow
See if the alarm now works , check garage keypad for water...