99 s80 interior light problem

How do you know it is the interior lights coming on? As for the alarm coming on, that is to prevent someone from stealing the vehicle. The siren has it's own battery back up in the event someone would try and kill off the battery to break in. It senses that is has being cut off and starts sounding off for help. Pretty smart of those engineers. But back to the root cause of your dilema. I would recommend you take it in to Volvo to have them monitor all the control modules to see which one is failing to go to sleep? Your problem can be from the Electrical Switch Module Courtesy Light all the way to the Central Electronic Module keeping it up. Some basic checks you can do for now is inspect the Central Electronic Module for water damage. Remove the sound proof panel on the drivers side and look up under the dash. See if there is any water leaking on to the module. when water gets in it will cause all kinds of electrical problems, like keeping headlights on when the car is off, difficulty starting, warning messages coming on. It was more common on the 2005-2006 models but still deserves a glance over. Other then that I strongly recommend you take it the dealership.