Seasonal Coolant leak

I'm having an issue with my 2002 540i. I live near Toronto Canada.

Every time the season changes between winter and spring and summer and fall my system drops coolant. A noticeable pool on the garage floor. This has been going on for years and the dealer says there is nothing that they can find that is wrong. Just had it checked again.

This car has a new rad, new expansion tank, new cap etc and nothing is leaking. Basically when we go from the really cold to the mild weather or from mild weather to really cold I lose about 300 ml of coolant. I can see it on the garage floor. Once the car issues the "Low Coolant" warning I add BMW coolant mixed with distilled water 40/60. I raise the coolant level to the proper level as measured by the float. I fill it once and only once per season change.

Everything runs fine and the discharge stops until the next season change. I wash the garage floor and everything drys out so I know this isn't an ongoing leak. Once the seasons change the coolant pool reappears and the process repeats.

Anyone with any ideas?