Thanks for the replies, o' Whiskers:
I'm connected to the car, I know exactly how it behaves and how it delicately sips oil,
I check oil-water at least once every 2 weeks, it was constantly full (slightly less after tune up but still close to max),
I check on level ground and cold,
But when I saw this I checked many times from 2 hours cold to 6, and moved parking 180 degrees to be sure.
It went in 3 weeks from almost max to ~ 3 mm below min., no smoke no leaks, no heating, no coolant change…
I don't know if to really worry or just slightly.
Of course I'll keep a tight eye, but I don't like it.