Hi,
I have the same issue - F10 523i. I have checked with a couple of workshops - and also some comments from others. It seems the F10 523i have this issue with the design of the Radiator unit - when travelling, due to the pressure; there will be leaks; thus; stationary when parked - we will not notice any 'wet patch' - and will wonder why every couple weeks need to top up (nearly 1-2 ltires of water into the Coolant Tank.
BMW seems to have noted this issue worldwide - and heard they have redesigned the 'newer' batches of F10 radiator to prevent this issue. I also checked - and it seems this is not covered under warranty somehow - but yet to be confirmed. Was told unless it leaks from the start - otherwise, its a 'wear' issue. Based on the info I got; I am not satisfied that this cannot be claimed under warranty if its really found to be true.
Still waiting for a full check to determine the confirmed source of the problem.