When you list a home, which has a security alarm, you need to share the code with the listing agent.  The listing agent will make a note of the code and place it in the lockbox where only a licensed agent has access to it.  This way, you can continue to use the alarm as usual and visiting agents will be able to enter the home without tripping the alarm.

The listing agent will also make a special note in the MLS listing, visible only to other real estate agents, indicating the presence of an alarm system and the location of the code, as well as instructions on the location of the panels in the home.