Explorer 2000 EB Fuel Cap warning light

On my Ford Explorer 2000 EB's dash board, the fuel cap warning light comes on when I start the engine then goes out (just as other warning lights on the dash)
BUT, it does not turn on if the fuel cap is NOT capped back on the fuel inlet.
During the last week twice I forgot to cap the fuel inlet after fueling, there was no warning light on the dash, I discovered my mistake after reaching the destination.
Where is the sensor for this light and where should I start looking to return it to normal function ?
Thanks in advance.
Matt
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