Coolant leaking on diesel engine

Hi, I have recently noticed that coolant was leaking on my diesel engine. -Coolant was between "MIN" and "MAX" marks when I left, no apparent leaks while driving, but when the car was stopped/cold, I noticed the coolant was down to the "MIN" value. There is no white exhaust smoke, the coolant hoses are not hard when the engine is running and hot. The car has no loss of power and seems to be running normally. At first the coolant leak was slow and I could not notice anything dripping under the car. Then with a coolant system pressure tester I put pressure at one bar, and the needle was dropping quickly. I could hear a big air leak at the right of the expansion vase. After putting the system under pressure, I started the car again and now I can see a leak clearly under the car to the ground at the left side of the engine opposite the expansion vase. I put paper under and the leak is clean/clear coolant. What could please be the cause? Thank you for your help.