Dunno about the nearest builiding problem. Presumably everything works fine if you try it with another building, the problem is just identifying this building, yes?
I don't know if this is possible, but if you could detect satchels going off you could do it that way instead.
Happily, I can help with the alarm. Make a seperate trigger that has the alarm sound, but does nothing else. Set it to Repeatedly
Condition: alarmOn
Use the variable alarmOn to contro the effect: when the variable is true, the alarm will sound. When it's false, it will not.