Run the car to the top on inspection and tap on the piston in several places to see if the noise changes(hollow vs. not). If 40 gallons of oil are in the piston you will know.

I have also seen what I call a flapper leak, the rust or corrosion is semi-sealing the leak most of the time and with full load it holds even better(until it doesn't). I have left cars like yours fully loaded after a relief test many weekends and not found any movement. Once I showed up on Monday morning and the car was in the exact same spot a few feet above the bottom landing(this test the piston joints as well) then we left to get a cup of coffee and call the boss. When we got back from coffee and talking to the property manager we were shocked to find the car on the springs and it wouldn't even pressurize the jack. When we pulled the cylinder out of the ground there was a rusted hole the size of a dime near the bottom of the jack. Scary Crap.