Fuel Gauge acting strange

I went out in the car today, it has sat unused on my driveway for the past week or so. We’ve had some torrential downpours for most of the time it was sitting idle. The car started first time and sounds normal, I drove onto the motorway then noticed the fuel gauge was reading over half full but when I’d parked the car the previous week it had less than a quarter tank. My heart sank, I immediately suspected the filler recess drain had become blocked and allowed several gallons of water into the tank, I turned back and headed home expecting the car to conk out before I made it back but it seemed to be running without fault.
I checked the drain in the filler recess and it was completely clear but I have been reluctant to risk driving it until today.
The weird thing is upon starting the car today the fuel gauge is now reading a quarter tank which is about right. So what caused the incorrect high reading ? If, as I first suspected enough rainwater had got into the tank to raise the level by a quarter would the car have still run without any symptoms ? And where has the water gone to allow the level to drop back down to a correct reading ? How can I tell if there is water in my tank ? At the moment I’m reluctant to drive it far until I get to the bottom of what caused the incorrect high reading. Any input or suggestions would be most welcome.

On my 1988, the fuel level transmitter at max level sends a ground to the cluster, minimum level = 5v. Maybe a wet connection somewhere in the circuit could have provided a better pathway. Knowing that fuel floats and the pump draws from the bottom of the tank, if you had that much water in the fuel, I would think you would have noticed the engine running rough at best!
You can add some water remover to the tank, but this is typically used to take care of moisture in the tank from condensation (low quantities). Again my 1988 has the fuel pump mounted external to the tank, under the car. If worried, I would clamp off the line, disconnect the feed from the tank to the pump, and collect 1/2 gallon or so in a clear container to allow for inspection.

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Hello Casso - tending to agree with both suggestions by Glen - if any water (from the rain) had gotten to any of the terminal connections for the fuel gauge sendor circuit, and then to a ground, this may have caused the erroneous reading - the water then may have been disperssed though the time of the engine running and therefore returning the cicuit to a more correct reading - as to checking for any water content in the fuel, the method Glenn states is the method used for any airplane preflight check to make sure no water in the fuel - Tex.