If the rear wheel is out of line the car will "crab" rather than pull. So the steering wheel will be slightly off but the car should track straight.
Wouldn't the caster in the front suspension cause it to pull in this situation. The front end should want to "straighten-out" from the caster, or is the caster angle so slight in the M3W that this is not a factor?
Since this is a new issue that the OP noted, just after a service, the most obvious thing to check would be the rear wheel alignment I'd think.
