Excellent read, thanks for putting this up. I will put forth my experience
1) I had a perennial problem with "low engine oil" level in my E90 (N46) engine
2) Despite of changing all gas kits and o rings, every month i was asked to add 1 ltr (1.06qt), i even changed the rear main seal and the car had to go thru a complete balancing, alignment process and BMW dealer charged me a bomb for this
3) No visible sign of oil leak, no blue flames or dark puddles or any symptom for internal oil leak
4) Drained the whole oil, and found that i had around 5 ltrs of it (5.3 qt), but level was still showing yellow (and asked me to add 1 more qt) - BMW manual says a max of 4.5 ltrs (4.7 qts) for the n46
5) Cleaned the sensor, changed the senor, still no luck
I was quite frustrated and was missing the dip stick badly.
One of the indy shop suggested me to apply sealant around where the sensor sits.
Boom!
its fixed the problem and now the same sensor and system showed the level above the max limit. Its been almost a year and time for new oil change and all is fine.
Punchline - I guess, some pressure leak effects how sensor reads the data, I am no expert, but I hope some may find this useful.
NB: the oil sensor area use to be moist, but it was hard to believe you are loosing 1 ltr from that every month.