I will check the battery situation as it seems very logical. For now I have checked the LSM module and it indeed was in fault with the headlights. I never used the switch itself much, as in Finland we tend to keep the headlights on all the time. I opened the LSM module and pressed the little buttons there in order to clean the contact points and it indeed worked - the headlights now turn off when the motor is off and work normally. You know Volvos are good cars when parts get better the more you use it
. About the oxygen sensor, I get about 29 MPG but I have never worried about it as the Volvo is known for it's consumption, but I guess it could be lower. In the MK2 model the battery is inside the passenger compartment, near the spare tire, but I will check the B+ terminal sides if there are any salt bridges, thanks for the heads up. Edit. some additional information