First of all I would retighten all hose connectors. Typical leaks are under the black plastic cover drivers side where the 2 heater valves are and underneath the aux water pump (on E32). The aux hot water pump could have a crack at the bottom where the motor is inside and you will not see the small leak. Best is you use a small dentist mirror on a wire with a small LED flaslight to see underneath, also under the water pump.
Another leak can occur on the upper plastic connector on the right side (driver side LHD) at the radiator.
Also check the return hose to the expansion tank under the cap for the expansion tank. There should be a constant flow back into the expansion tank from the radiator.
You will notice that the aux water pump is out of order when the heat gets less when car runs in idle at a red signal or in traffic jam, when the RPM is low and the main water pump does not circulate so much coolant.