Slow coolant leak?

[Chucksndunks]

I bought a new to me 2017 m3 a little over a year ago, about six month ago I got a low coolant warning on the dash. I checked the Inter cooler reservoir and it was fine. The 2nd larger resevior was low so I topped it off. Now, just a couple days ago I got the warning again and same issue the engine coolant was low. I have never been able to spot a leak or left any liquid after been parked for days. Never noticed any coolant burning smells or dry coolant in the engine bay. Any ideas or direction on how to diagnose ?

[Kilabyte]

Check the seams on the larger tank, these tend to split, it's a poor design.

[mapster37]

best way to diagnose is to top off the reservoir, then pressure test the cooling system.

[FuRoRR]

Coolant contains water and 6 months are a long time. If the car gets driven like it should it may get hot enough to open a valve in the coolant expansion cap that vents the overpressure in the coolant system to the atmosphere.The cap obviously lets some water vapour pass thru ,hence the lowering of the coolant level.
If you have ruled out leaks (which are relatively easy to see on a regular ispection) this is the explanation of your low coolant level.