Basic Troubleshooting and Reset
A single faulty sensor can result in the alarm system acting erratically. Turn off the system and walk around the house to visibly check each sensor on your doors and windows. A sensor has two parts: the transmitter and the magnet. The transmitter should be on the frame of the door or window, while the magnet is on the door or window itself. If either the magnet or transmitter is starting to slip out of alignment, or if the transmitter is starting to fail, the alarm will start going off at random times. Replacing the sensor can correct the problem. Check the batteries as well of any devices that use them to ensure that everything is properly powered. Once you've done a walk-through and confirmed the sensors are properly in place and appear functional, reset your system by turning it back on and hitting the reset button on the control panel. You may need to enter your access code to complete the reset.
Examining the Panel
Security systems are designed to self-assess so that they can alert you to potential problems. If the system is registering a glitch, the control panel will often show an error code on its display. The panel is not large enough to allow a full explanation of what the error is, so panels will use abbreviated codes to show the problems they are registering. Check your manual for a list of trouble codes and what they mean. Common error messages include a communication error between the security system and the phone line, a power failure to a security device, a sensor problem, battery issues and the locations of recent alarms. The codes for these errors will vary per system model.
Many alarm systems are professionally monitored by alarm companies that receive notices of both alarm activation and possible system error messages. If you're having trouble with your system and can't figure it out, you could get remote assistance from your alarm company. The company operator could likely diagnose the problem over the phone, based on what you describe and what the operator is seeing on the monitor. Know your security contract in regard to any fees that apply to service calls and how long your system warranty lasts.
Another option for troubleshooting the system is to put it in "test" mode. If you are using an alarm monitoring service, call them and request to place the system in test, which prevents alarms from being sent to the police department and will allow the system to activate without being credited for a false alarm. If your system isn't monitored, there should be a testing mode that you can access through the panel, though this varies depending on your system. Putting the system in test allows you walk through the house and test each sensor and device individually to see what's working and what isn't. Notify your neighbors before testing the system, because your siren or other audible warning could go off during the test, and the neighbors could be startled and call police.