Fluid and hydraulic problems like this can be very frustrating. Your braking system has moving seals, so static testing may not reveal all. I would put first trust in a pressure test if competently done, tricky though, because seals might get better according to the direction of pressure.
Fluid loss can be in 2 ways - the standing head of the reservoir and fairly large hole somewhere, or fluid loss from a very small hole or seal on the high pressure side somewhere, which would only occur when braking. On the latter, if you applied the brake on the stand for several minutes with motor running, I'd expect the pressure to sag
You need to get more into the sytem, but pressure tests might need to be done in 2 directions - at the filler cap and at the bleed nipples with a sealed reservoir cap. One other possibility is an air lock in the system between the reservoir and first lump of hydraulics. When bled the system could work perfectly as there's not much fluid drop needed in the reservoir for braking. With positive pressure testing, there is a chance of trapped air being introduced afterwards and the system should be fully bled through. If you are replacing a reservoir quantity of fluid on a regular basis then any airlock would have moved through.
I haven't delved into these ABS systems, but check if there's a possibility of a seal leak and fluid accumulating in a void somewhere. Similar to what might happen in an enclosed clutch housing.
I don't know what BMW's pressure testing procedure is. Is it a test of braking fluid pressure or designed to detect leaks? It might just be a guage on a nipple with brake applied. That wouldn't find a reservoir leak. If a low test pressure e.g about 1/2 psi from a cycle pump and a water guage manometer were left on a sealed reservoir cap for 1/2 hour I'd expect no negligible pressure drop if there are no leaks.
One thing for sure, we don't get diminishing fluid levels in our brake reservoirs - so you are right to look at where it's going. However, you could consider the following:
Fill up the reservoir then date and measure the level before and after each ride. By looking at your log you can work out if any fluid loss is greater when the bike is standing or ridden. That could eliminate the reservoir lines and you'd be looking more for leaks around the working parts of the system. Do the same thing with the motor running on the stand, aggressively applying brakes for long and short periods. The only way you may find a leak with the clean paper method is to excercise the system. Leaks under gravity alone will be very small and hard to trace. When I'm looking for any fluid leaks I use clean white tissue paper. Wrap a turn or two around each nipple or junction and the hydraulic components you can get at before starting the aggressive stand testing. Good Luck.
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