2006 Chevrolet Malibu 4 Cyl Having Fill Coolant Every Days There Leaks

I have a 2006 chevrolet malibu 4 cyl. I have a problem with having to fill up my coolant every 4 days. There are no leaks that i can see and no white smoke out the tail pipe. I took it to a dealership in my town and they checked it by doing a pressure check when engine was hot and cold. They found no cracks in the hoses, no cracks in the resovior, checked the heater coil, spark plugs, floor board to make sure there were no leaks, checked my oil. They want me to check it for 2 weeks. 4 days after the dealership filled it up ....i had to refill with coolant again 4 days later and didn't drive over 150 miles. Anyone tell me what is going on and why they cant find the problem