I'll take an experienced guess: the leak is on the coolant reservoir right around where it branches of for the return line of the cabin heater. It's a stress fracture on the seam of the mold. You don't normally see since water seeps out only when the system is pressurized. Tell the tech to warm up the car and disturb that general area by applying pressure to the reservoir or gently wiggling the coolant return line.
2003 BMW Z4 2.5i
2009 BMW X5 xDrive35d
2006 Ducati 999S
2012 Ram 2500 Laramie Longhorn 6.7 Cummins