coolant level ding, when does OBD activate

[omodos]

I read somewhere that even if the rod is at min, there is still plenty of coolant, and the moment to start worrying is if the OBD starts ringing! At what point the OBD activate though? Is it when the min level is reached with the rod, or way below that level?

I have the mystery coolant loss issue, every 1000miles of combined urban and highway driving I have to top up from min to max, it only takes a small cup to get it to max, surely this minimal loss in such a large coolant capacity system cannot cause an issue so why is the level min set to min?or is it just to play it safe in bmw's part?