I had a similar problem on my previous 2005 S40 2.0D. I kept getting the 'alarm triggered' message. Sometimes I did hear it go off, others I didn't.
Seemed worse on a hot summers day (remember them?!)
Whilst I never had an issue with the volume of the siren I couldn't explain the messages. Turned out it was the air mass movement sensor in the cabin that was faulty - attached to the rear of a squarish blank looking plate in your headlining, between the front and rear seats in the middle of the roof.
Some were over sensitive from the factory and were replaced under warranty. Have heard of similar issues on the V70 and its in the same place.