| When i got mine at 80k miles i gave it a once over. all seemed fine.
checked two weeks later and the expansion tank had coolant stains down it.
Water level was low, but it never over heated.
I bought a new BMW cap and replaced the coolant. Still ok now, but i'll be keeping an eye on it in the future.
I think if i had not noticed the stains then the car would have soon overheated and once that happens it's too late for some engines!
probably worth swapping as an annual item given the risk factor. | 
