Moisture under floor mat

[Scottsdale1977]

Hello my 1977 scottsdale that I have restored and keep in the garage for some reason keeps getting moisture under the rubber mat. Several times I pulled up the mat and there is moisture under it. I just usually dry the floor pans then reinstall the mat. The truck isn't drivin in the rain at all and I've even checked after I washed it and can't find any water leaks since this is the only time my truck usually gets wet. Has anyone had a similar issue? May this is one of the reasons the floor pans rust out on these trucks.
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