There was the apartment robbery, ..
RSBM
I don't know anything about this event, but was it a robbery or burglary?
Legally, a robbery involves confronting /threatening a person with a weapon in order to commit theft; burglary is breaking into a home when no one is there in order to commit theft.
I assume the incident was burglary. I think theft is common in peaceful suburbs, typically young people lurking around to quickly grab something they can sell (often to buy drugs). Bikes, tools, electronics, a wallet. They have no interest in robbery, have no weapon, etc.
Such a person might easily sneak into a house while the owner is preoccupied with a garage sale. Grab something and be gone. I think that's why she would set the alarm.
Actually, it's more likely she just always, automatically, set the alarm. If you only do it sometimes, you're more likely to forget when it matters.
Then you would always, automatically, have to disarm it before you enter. Again, that way, you're less likely to forget and accidentally trip the alarm.
Even if you do trip it accidentally, you have time to reverse it before it goes to the alarm company.
Only if it goes off, and a considerable amount of time passes without it being corrected, does the alert go out to homeowners/backup.
People I've known with that type of system are not lackadaisical about turning off the alarm before the alert to the alarm company/family members gets triggered. It only takes a moment. It makes a loud, urgent noise. Everyone will get on your back if you're negligent. If for some reason you miss the window, you contact everyone to say it's a false alarm.
If, say, Liz and S had left on their trip, and an alert was sent to the parents about the alarm, they'd need to get to the house to see whether it was burglary, so that they could call police. Again, that's a pretty big responsibility, if you weren't willing to do that, you'd better not agree to receive the alert.
However, in this instance, they knew Liz was supposed to be home. Why would she allow the alarm to go on that long without turning it off? Why hadn't she texted them?
In their situation, I too would have sensed something was seriously wrong, and since they were very close by, just head there.
JMO