Try what letmetellu suggested... if you hear water charge into the tank when you turn the water back on then the leak is confirmed... although I think issue now is to find it as we're both pretty sure you have an underground leak somewhere.

OK, I turn the water line off at the top.

OK... give it 3 minutes and then go turn it back on... if water rushes in (it shouldn't)... leak confirmed... just need to find it now!

Bad news!!
Yes I hear the rush of water going into the tank
Thanks for the help!!

Glad to help here...

For tonight I'd shut off the water into the heater and turn the gas valve/thermostat to pilot. Turn all on in the AM when needed.

Good luck... hope it's found easily!

Me to.