On mine X540 the deck will drop all the way over night. It will drop even if I am doing other work with the tractor like pulling a cart over a period of say few hours...I hit the lift lever and the deck will come up maybe a 1/2 an inch. Just enough to see it move.
I had the same ? a few weeks ago and mine will leak down overnight also but no when the motor is running. I will be dumping Amsoil in my tranny right before winter as I should have 25-30 hours on by then.