Why coolant is leaking

[325iZYX]

Hi,
I have 2001 325i, changed the water pump and thermostat last year. A week ago expansion tank was leaking and notice a crack. Replace the expansion tank and coolant. I fill with 50/50 mix (BMW coolant and Distilled water) and when engine was cool opened the expansion tank cap and checked coolant level 3 times after running the engine for 10 minutes while revved the engine with heat on hi. Later, I have run the engine for 30 minutes and water temperature position is at 12 o'clock. I went for 15 minutes’ drive the water temperature position was still at 12 o'clock, however coolant sensor came on indicating I was low on coolant. I notice it was leaking coolant on the same side as expansion tank. I double checked everything and did not see any cracks and all caps and blue screws were tight. Any idea why coolant is leaking? Thank You All.