The difficulty of getting to the wires will depend on the quality of the install.... And how well wires are routed / hidden behind wall units and such like...
If the system is any good the siren will have its own battery once communication between the remote key pad or pir is lost with the alarm set it should scream the place down....
Biggest vunrability in my mind is the siren grill know from old house alarms a can of expanding gap fill foam mutes sirens rather too well...
Likewise make the siren / control box very hard to get at from the inside so you can't just smack it with a hammer to silence it
Anyone entering an alarm protected property would set off the alarm , as would cutting a wire , almost everyone hearing an alarm go off regard it as a nuisance , so don't expect any response , unless your paying a security company to monitor it for you , my best advice would be to have the alarm linked to a phone response unit , that will automatically call a few numbers , and deliver a set message to those numbers .
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