I think this might be a common theme on our cars... I get the check coolant level occasionally (maybe once every month to 2 months when there is drastic change in temperature). The dealer said that over time with the amount of miles that I drive (1000 kms a week) a bit of coolant can evaporate and a top-up is not unusual (my top ups are generally about 100-200ml... not much at all).
I have had checks done for leaks at multiple shops including the dealer with no evidence of leaks. My expansion tank has been replaced as well.
2015 M5 Competition+Ultimate Package black sapphire on champagne
2003 540i/6 black sapphire on black
BBS RG-R, CDV, Dinan Exhaust, Dinan RSB, Dinan RSTB, Dinan SSK, Dinan FSTB, Dinan CAI, Dinan TB, Dinan MAF, Dinan stage 4 software
BMW 13th gen ULF Bluetooth, BMW Sirius, Dice Silverline iPod
Dynocom numbers: 320HP, 420LB.FT TQ (crank)