Add a "one shot" module. Any alarm parts store should be able to help you.
It goes between your circuits and the bell/siren/sounder outside the protected premises. No matter what your circuits send... they might mis-behave, for instance... the one shot module ensures that the alarm only rings for a time set by you... 15 minutes? Your neighbors won't be pleased if your alarm goes off at 2am the weekend you are away, but they might forgive you if it shuts up after a reasonable time period.
Also add, if not already in your plans, an LOUD interior sounder. I was advised by police that villains are sometimes persuaded to leave early if there is a nasty, loud, sound inside the building they are "visiting". Villains. Not police.
More on alarm system programming, not Arduino based, but still relevant, at...