I think it is unlikely that you have a tiny leak in the water heater. I agree with LTC - probably a fitting somewhere. It is cheap (though annoying) to replace these, so just go to it. To my knowledge, there is no history of leaks in the water heater itself.
Do things freeze in your part of Texas? A freeze of a small amount of water trapped in a fitting can cause a tiny, almost invisible, split in a plastic fitting. Finding such a split is nearly impossible. Easier just to replace them.
Bill
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