2001 SAAB 9-3 Coolant leak

Replaced radiator, found leak in heater hoses replaced.
Fill coolant still comes on, losing about 16oz every 20 miles, fill reservoir, fill coolant screen comes up. Mechanic has run at idle for 45 mins, run at speed on street, put pressure gauge on coolant system put at 20 lbs, holds pressure, test pressurized reservoir cap and replaced. Put on rack, can't see any leaks, drive 15-20 miles, fill coolAnt screen comes on. Have laid in front of car after driving 20mins, continue to watch while idling, no leaks apparent, while mechanic puts pressure gauge on still no liquid dripping. Car has never overheated.