losing co2 but holding pressure

I set up my new kegging system very recently but I lost my first bottle of co2 to a leak. I got a new bottle and checked all the connections with soapy water, turns out my regulator was leaking. Fixed the connection with some teflon tape and it's not leaking anymore. I turned on the gas on a keg of some beer that was previously carbed and the next morning the pressure on the secondary valve had gone from 800 to near 400 (in the "ORDER GAS!!!!" zone). I shut off the tank to see if it was holding pressure and 3 days later it still is at the 10psi. I'm stumped as to where the gas is going!! Any ideas???

Big time temperature swing? Are you sure you are checking the pressure in the system fine and there are no check valves anywhere?

Seriously, if you want to know exactly how much gas you have left, weight your bottle (You'll need to weight it empty too if it doesn't have a tare weight on it). 400psi might just be because the bottle is very cold (in the fridge?) and it actually has lots of CO2 left in it.

Did you put the bottle in the frig between reading 800 and 400 psi? A warm co2 bottle and cold co2 bottle will register much different pressures even though they have the same amount of co2.

900psi is average for room temperature CO2

400-500psi is average for fridge/kegerator/keezer temperatures.
pressure decreases as temperature decreases.

you don't have a leak. if the cylinder was empty it'd be almost 0psi. as long as there's liquid inside, you get normal pressures.