Fortunately I am well experienced with the "plumbing" of the coolant & radiator system - having replaced all 5 Ignition coils myself.
I did a 600 Km trip, and parking at one 'view point', I noticed a small puddle of +/- 5 cm in diameter, developed under the bike. The outside fairing area of the right "flooter light" had the tell-tale sticky water traces of anti-freeze. Anti-freeze doesn't evaporate as quickly as water...so lots of tell-tales.
I did not refill, top-up the reservoir..... and completed the last 100 Km "as is".... no more water puddles and the temp gauge was 'dead center' all the way!
Still not having topped-up, I did the return 600 Km trip with out a hitch. Temp gauge perfect!
Once I got home - a serious trickle of water ran out from the lower right fairing... I'd guess a full cup... as soon as I switched the motor off. I assume no airflow, water pump to cool the water = pressure build up...
I realized it could only be the radiator cap; were it a hose / pipe that 'popped', the internal water pressure would have;
1. Drained the entire engine, which holds a good few liters.... +/- 5 L(?), in a matter of seconds
2. Temp gauge display would have lit up like a Christmas tree - with plenty warnings *which it never did*!
(Those who haven't bled their water systems properly will know *exactly* what I am talking about - the Xmas tree display!!
Good to know; it seems although the cap failed, there was still sufficient coolant remaining inside the radiator/engine, to cool the engine....
Final analysis : Radiator cap not withholding (capping) extreme temp & pressure build-up... opening and venting up into the reservoir bottle, which has a rubber "teat" with a hole in it - and expelling radiator fluid.
(New cap ~ $35 expense.)
Mileage : 76,000 Km. Oh man - these ignition coils: last to fail was at about 68,000 km. (Fortunately BMW Coils have a 2 year warranty on them, but still, to replace them : schlep of note!)
Refilled with anti-freeze mix and now, 5 weeks on : all is good!