That is around the mileage I was seeing when my Vanos solenoids went out. They did eventually trigger a code/limp mode but it took about a week for that to happen. Have you scanned for codes? Not everything triggers a half engine light.
If you were losing fuel my guess is that you would be able to smell/see it at that rate. Warranty?
2008 E90 335i // Black Everything // photos
COBB tuned / QTP e-cutouts / Helix FMIC / cp-e DCI / AWE BOVs / Black M6 wheels + Toyo Proxes
(and yes, I am a girl)