I work as a tech in south Miami. I will use regular water to top up a coolant system of a car if I open it and find that the level is just below minimum. If its more then that I'll charge 1 qt of coolant (which BMW does make and sell) and then mix it with water to get the car topped up to the right level. Also I highly doubt you would of been able to drive the car 3 days with a loose coolant cap bleeding coolant. You would of seen the coolant beneath your car and been smelling that sweet/tingy smell that hot coolant has. What it sounds like to me is that your thermostat may have taken a dirt nap. Unless the tech that performed your oil service was in and around your engine bay somewhere he had no business being for an oil service I don't see how it could of caused what your having now.
Post up when you find out what's wrong with your car, I'd like to know. Pm me if you have any tech questions
