My understanding may be wrong, but somewhere I recall reading the outside alarm would automatically stop after 20 mins, the inside panic alarm would continue until turned off.
So I don't know where any of the ideas about the killer 'turning off the alarm' come from.
Similarly, somewhere we apparently have a witness claimed to have heard the outdoor alarm around 10. The normal thing - I can testify being a neighbour to someone with an alarm - is you assume the neighbour has accidentally triggered it themselves. You certainly don't call police and waste everybodies time...In fact, police refuse to answer home alarms, it's almost always false. Just like car alarms.
JMO