I currently have a 2010 S40 T5 and noticed a battery drain problem. I had the battery replaced last year and with the new battery, I have been keeping a close eye on the drain. While this may not be 100% accurate, I use "Automatic" to track my car's information. One of the features they have under "labs" is the Battery Health monitor.
Anyway, 2 weeks ago when I park my car in the garage, I stopped alarming my car. Basically, I would just let the car sit in the garage with the alarm "off". I would say the majority of any car owner would do this since the car is already in the house. So I did this for all of week 1. For week 2, I started to set my car alarm to "on" after I'd park my car in the garage. Based on the Battery Health monitor, there seems to be a substantial battery drain when the alarm is "off" versus "on". In the attached picture, week 2 starts on April 14th.
Can anyone verify that with the alarm "off" or "on" can cause noticeable batter drain while the car is turned off? The huge spike going down towards the 50% mark are times when I parked in the garage with alarm off. For the spikes that are healthy prior to 4/14, they were when I was parked at work with the alarm "on". After 4/14, I continuously set my alarm to "on" wherever I went including parking overnight in my own garage.