I have a 2004 X5 E53 3.0i and I know that it should take more then 6.5 liters or 6.8 quarts to fill after an oil/filter change.
It should be close to 7 - 8 quarts or 6.6 - 7.5 liters to fill approximately.
If not, you would get the lower oil line just half way below the fill as it needs enough to fill the filter etc. If not, double checked the oil when filling after starting the engine on level ground. Not checking after driven many miles/Km's without checking could initiate such a find.
It's not uncommon after 8700-9000 Km or 5400-5500 miles to lose a quart of oil or so. I check and change mine often and have found what your concerned about once before and even talked to a BMW mechanic about my concern as well. This is what I was told several times, so If I missed something, ECSTuning, please let me know.
It's possible that it could be burning elsewhere, so check for smoke as suggested. If no leaks, could be a simple mistake? Unless described above from ECS tuning.
Last edited by Mudnyrey; 11-14-2012 at 01:28 PM.