Pardon me for being fixated on the open doors. Certainly they understand an open door? And understood it enought to shut it, but were not able to comprehend that a blinking alarm and an open door (twice) might warrant some investigation especially at 1:30 (when Damon alleges he saw the open door and the alarm,and shut it) and later (2:30-ish)?
I am just GUESSING,but I suspect that since there were other INVITED guests inside the house,they may have thought somebody innocent opened the door. Even if the rumors about the party in the garage are true,I think it is safe to assume that people were leaving the garage at various times to get a drink,use the toilet,or even step outside for a breath of fresh air. This is NOT a high-crime neighborhood they live in,and they didn't worry about it.
Like I said above,I don't KNOW any of this. I'm just guessing.