2 months and 10 days since last fill of coolant, today I checked out of curiosity (before the low coolant light comes up), and it's low. I added 2 small cups of coolant and 2 small cups of water to have it back to the min level.
I do notice that X3's coolant cap takes no effort at all to open (compared to my E90 which is very tight), I wonder if it's being evaporated (that's why it's not showing up on pressure test?).
I did not touch coolant on E90 for 2 years and it's still the same level.