I had battery drain on my volvo c70 2001 for a number of months which was driving me mad would use it for work done home park up and next day wouldn't start. Had to charge the battery.
In desperation looked on the forum for this and discovered others were tearing their hair out with this too.
it has now been fixed . Turned out the problem was with the alternator. There are diodes which are wired in series parallel with DC output which allows energy to be charged to the battery in one direction only if the diodes are faulty there will be a DC f feedback which will subsequently drain the battery.. Remedy new alternator or possibly strip down as I did and replace the diodes but use good parts.

 

curious if the problem was in the voltage regulator vs the alternator. On many cars like my VW CC :-( the regulator is soldered on, so if an electrical component like a diode or in my car's case, a charging relay fails, you need a whole new unit for $500+. Good news on the 850s and early year V70/S70/C70s the voltage regulators bolt on and can be replaced without removing the alternator and the regulator goes for about $50...

 

In our case the issue was caused by an alarm module located in the engine compartment right under the windshield cowling, passenger side. Replaced the module and no more battery drain issues.

 

I have a 2006 s60, and am having the same problem. Battery kept draining out to dead. Bought a new battery a month ago, and it was dead after sitting for about 12 days. I have been seeing "Alarm Service Required" message for like a year now, but always just ignored it, as the alarm is irrelevant to me. I'm taking it to the mechanic this week. Does the Alarm Module contain a small battery that gets charged by the main battery? From some Googling, it's my impression that if the Alarm Module battery won't take a charge anymore, the Main battery will still keep trying to charge it, thus draining itself. I'd be happy to just pull the fuse on the alarm, and leave it at that!