if they pressure tested the system, then the system is fine (sans cap unless they also tested) and more than likely it's not disappearing into the motor.
Remember, the cooling system on the M96 is ~ 5gal (varies slightly depending on motor but w/ a '99 IIRC it just about 5gal). So, how much are you always adding to it? I'd say if it's been more than 3qts (cumulative), you might have an issue someplace...
Otherwise, have you tried burping the system yourself? Here's the procedure courtesy of Pedro:
• Start with a cold engine.
• Remove both caps (oil and coolant).
• Pull bottom plastic cover out (using a spatula lift one edge and pull out).
• Lift bow on bleeder valve. Screw oil cap on again.
Note:
On Tiptronic vehicles, the pneumatically operated coolant shutoff valve (flat-seat-valve) must be opened for the bleeding process. The electric switch-over valve must be switched off for this purpose.
Triggering can be performed with the PST-2 or PIWIS, or by pulling out the Tiptronic control module fuse B1 with the ignition switched off.
• Check whether coolant shutoff valve is open.
• Fill reservoir with coolant up to the bottom edge of the filler neck.
• Run the engine at idle and top off with coolant until no more coolant flows into the cooling system when the engine is revved moderately. (Coolant level now at the lower edge of the filler neck) The coolant temperature must not exceed 80 degrees Celsius; proceed directly to the next step if necessary.
• Close the reservoir and warm the engine up to operating temperature at an engine speed of 2,500 rpm until the thermostat opens (approximately after 10 minutes). Coolant temperature should now be 90 degrees Celsius.
• Check: The radiator supply lines and return lines in the front wheel housings must be hot.
• Allow the engine to run another 5 minutes at 2,500 rpm. Every 30 seconds, briefly rev the engine to 5,000 rpm. This is important for proper bleeding.
Note:
The level warning light can light up during the bleeding process (reset by restarting the engine). Interrupt the procedure if the coolant level is below the "MIN" mark. Open the reservoir with extreme caution and allow overpressure to escapeuntil the coolant level reaches the lower edge of the filler neck; top off with coolant if necessary. Again, intermitently rev the engine for 5 minutes.
• Allow engine to run at idle speed until any activated radiator fans switch off again. Then switch off the engine. Open reservoir cap with extreme caution until the reservoir is depressurized (danger of scalding!).
• Top up coolant expansion tank until level reaches lower edge of filler neck (cold, this corresponds to = "MAX").
• Reposition bow on bleeder shutoff valve. Unscrew cap.
• Put on cover and body seal. Screw on both caps.
• Reactivate ATF shut-off valve on Tiptronic vehicles.
Also, when you remove the caps and find the bleeder valve, is there any white residue residue around it?
What are the last 2 digits on your coolant tank cap?
Good luck
PS - the cheater method to burping the system is to pull up the bleeder valve and drive around for 20-30 min with it up. Not as effective but can work in a pinch.