Well, Matrix failed is notice check (I presume), so he's not really sure what's going on inside. There's a voice calling the heroes out, so he would figure someone
is inside, even if no one can see him just yet. By activating the security system, he's hoping that a) whoever is inside will trigger the alarm, and b) the alarms will momentarily spook whoever is inside so someone else can catch him off-guard.
However, you're right about the door being open so that wouldn't really work. Matrix was on the side of the house so we can chock that up to him being ignorant of the particulars of the situation. Eventually, he'll get antsy enough and probably send his board through the window or something.