In the first place, if I am investigating an alarm at home, I will have a firearm either in my hand, or holstered on my hip, regardless of the time, ie. day or night.
In the second place, if I saw someone breaking into my car or my neighbors car, if they were not aware of my presence, I would try to call 911 and get the police enroute -without alerting them to my presence- or the fact that they had been "made"... while waiting and watching, I would try to be a "good witness" and make note of anything - descriptions, clothing, license numbers, etc that might help the police later. While watching, I would also be making a threat assessment - any weapons visible? age, size, fitness, how many of them are there, etc. of the perpetrators, etc.
If the perpetrators became aware of my presence at any point, I would have to make a split second decision of the best course of action, based on several factors: are weapons visible? are the perpetrators concealing something they don't want me to see (ie. possible weapons)? are they leaving or heading toward me?
Ideally when they became aware of my presence, I would secure myself in my home, barricade myself and maintain contact with 911 operator until police arrive. If the perpetrators rush toward me before I can withdraw and securely barricade myself, and depending on their specific actions and whether they had weapons and if so, what type and how many, I would have to make split second decisions, based on the factors present at the time, of whether my best option was to retreat and attempt to secure myself indoors or take cover and prepare to fight.