[diro]

Your horn and hazard lights are controlled by CEM.
Poor battery can kill CEM or do harm to it. If you had undervoltage before that could affect CEM and you have problems with your alarm system. Drivers get messed up in the CEM
Its not sunroof blocked drains what cause CEM water damage as someone mentioned before - its CEM's top wiring inlet rubber cover and lid gasket. You can have best possible sunroof drainage in your car but you will always get water in the drainage pan/tray or whatever you call it. It is under wipers top cover - panel. That panel is attached to the windscreen and covers wipers mechanism and wiring. Once panel is removed you can clearly see water drainage tray. Thats the place where the most of the water gathers under heavy rain or car beeing in car wash facilities. On boths of the sides of the tray there are water drains. These may get blocked by bits of leaves and other rubbish during a time. That blockage can cause a very quick water acumullation in the tray and water may get into CEM terminals if lid gasket and wiing rubber covers are irritated and dont seal properly.
I would suggest you to inspect CEM. Even if terminals OK it still can be faulty. This is one of the most unreliable electronic units in xc90 along with the multimedia system - centre console