Many MINIs came from the factory with air pockets in the coolant system which burp and then the coolant level goes down. That said, you should bring it in to the dealer for a pressure test to make sure it isn't something more serious. If the pressure test comes out negative then they should top off the coolant for you and you'll be fine -- may need to top if off again but as long as the pressure test is performed you can sleep well.
Just top it up with clean water when the engine is cold.
Keep an eye on the level and if it continues to diappear then you know you have a problem.
My first cooper used a little coolant during the running in period and lost non for the next 10000 miles