Common places for loss if you genuinely don't have any external leaks are the valve guide seals and the CCV. Valve guides can be done without taking the head off but its a long old job. There's some how-tos on youtube if you feel like a challenge. Be aware though that it would be relatively easy to drop a valve while you are doing it, after which you are almost certainly taking the top off. My local indy quoted a ballpark of 1500 quid to do my 525i but he was guessing since he wasn't sure quite how long it would take. Annoying thing is the seal kits are dirt cheap.
Before you get into that though get the car up on a ramp, get the tray off the bottom and have a good look around. It might be a small leak dripping onto the pipes and burning off before it hits the deck so not obvious.
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