Wow, that's very bold of them to try to break in during day time, particularly a Saturday when many people were home they would have heard any strange sound.
My thought is, someone would have to know that you have a home alarm, and that you would leave at certain time on a Saturday. I mean I don't think this was not lucky pick. Without knowing these information they would have to:
1. Knock at each unit to find out if anyone was home 2. Find out if the home has an alarm system.
Think back to see if you have recently hired a contractor to work on your home and would know these information. Two bedroom in our complex is hard to break in, because they would have to jump over the fence to get to the glass doors. If you put a stick behind the door's track then no way they could get in. I've heard of several break-ins on units that have window at the corner. Other than jumping the patio fence, a second obvious action is to shut down the electricity, a few homeowners on that building could have gone out to investigate. There were just too many reasons to point out that your unit was a planned target and that they could make a hit and get out within a few minutes.
There was an incident for my own unit some years ago. I was at home around 9 PM and heard a push at my front door, my old door always had that thump sound when someone pushes hard on it. I immediately came out with a gun in my pocket (I have a Concealed Handgun License), I knew that person would not be able to get away that fast. I came out and saw no one. I looked toward the direction of my other patio figuring the only way that person could get away that fast was to hide behind that patio fence. Sure enough, a hispanic male emerged, walking toward me looking like he belongs in the complex and said hi to me. I had my finger in my pocket and on the trigger, ready if he'd acted in any threatening fashion. He walked toward the mailbox area and gone. I would not advise anyone to do the same thing I did.
Thanks Amy for warning us. I will be sure to secure my glass doors and may add one more deadbolt lock to my front door. My advise to you is to do the same. Don't trust the alarm as the backup battery would ony last an hour or so. By the way, please try to keep your garage door closed at all times.
By the way, sorry for all that deleted posts, I just don't like my typos. I wish they have edit mode.