Why hasn't there been any water in my car tank, and I haven't seen any leakage there?

You said you couldn't see water in the water tank and you couldn't see it in the kettle. In fact, there may be water in the water tank, but some of it is missing. If no obvious water leakage point is found underground, this water may leak into the engine and flow into the fuel tank. Unscrew the oil cover to see if there are any traces of oil emulsification (punch or pin corrosion). If there is, it means that the waterway and the oil circuit are connected. You should go to the repair shop to check, remove the engine and replace the cylinder gasket.