Water heater leak troubleshooting for dummies

[Thaplumbr]

Sounds like a pinhole in the tank from the information you've given, when the supply valve to the tank is open and no fixtures are being used, there is standing pressure on the tank, in my neighborhood, we typically have about 70 psi from the utility, also this time of year you will have condensation on the supply line if its not insulated!
Just my
Leroy