Having a home security system is a fantastic way to keep your home safe. But it can also be a bear if you have housemates who all have different schedules. If your home has the type of alarm where any movement inside the home sets off the alarm, you have to be careful because you never know if someone is home and merely napping. The risk of turning on the in-house alarm when someone is still home is very high, and that can result in a major shock and a possible police call that are unnecessary. Given that many cities are starting to fine people who have false alarm calls, it's best to get a protocol in place for when to turn on the alarm.
Check Parking Spots
If you're about to leave the house and think everyone is gone but are not sure, check the various parking spots around the house. Note that differing traffic levels often force people to park a block away or so, so take a quick look. If you have adult housemates with different schedules, you don't want to risk setting the alarm only to find that someone was home and asleep -- they'd get the shock of their lives when they started moving through the house.
Assign an Alarm Supervisor
It sounds silly, but sometimes the easiest thing to do is assign someone to be in charge of the alarm. This should be someone who is generally in good contact with everyone in the house; at the very least, everyone should have the person's phone number so they can text the person if they're going to sleep or be out. For example, if someone who usually parks in front of the house has to park a few blocks away, it might look to others like that person isn't home. However, if the person got home before everyone else and went to sleep, and then others came home and decided to go out again, those others might set the alarm, not realizing that the first person was actually home. By having one person in charge of the alarm, the person who got home first can text that person to let them know that they are home and are going to sleep. The rest of the housemates won't have to worry about setting the alarm.
Change the Alarm Type
Another option is to change the type of alarm you have from one that detects interior motion to one that detects perimeter motion. That way, anyone in the house is safe from setting off a false alarm; they just need to remember to check the alarm status before opening a window.
Alarm companies have different systems for different situations. You'd do well to talk to these companies to see which types of alarms might fit your household's schedule the best.