Puzzled! Where did the CO2 go?

I had  500 lbs pressure yesterday and nothing this morning. My keg had been holding pressure for more than a week and I had tested the entire system with star san around the fittings to catch any leaks, and everything had been looking ok.  Fine one day, a bust the next.

Obviously I have a leak somewhere, but what is really a PITA is I’ll have to wait until Monday to refill the CO2 bottles because all the suppliers are closed until then.

Has this happened to any of you before and if so how often?

You do know that the pressure in the keg is not an accurate measurement of the weight of CO2?  Perhaps it was very low.
Still, you need to find a leak.
I am so paranoid of leaks that I close the tank valve after every use.  I don’t even use the “set it and forget it” method of carbonating for fear of emptying a tank.

Both the CO2 bottle pressure on the regulator and the gauge for the keg read zero.

Yes, of course it’s empty. I was talking about the pressure you mentioned before that.  The tank pressure doesn’t change much between full and almost empty.

In the tank it is CO2 Gas on top of liquid CO2 at the same pressure. Once the liquid is gone, the pressure drops very quickly.