One tiny brown spot on one leaf on one plant grown hydroponically?

Sorry, haven't a clue.

If the leaf bothers you, snip it off. Also, nute burn is a possibility. Your nutes could be piling up faster than they are being used.

Are you topping off your reservoir when needed or are you doing full changeouts? If you're topping off, are you using plain water or are you mixing in nutes? Has there been any changes in light or heat/ duration or intensity?

To be honest, what is shown in the pic, I wouldn't really see as a cause for concern yet.

We've had a lot of rain recently so the humidity has been high. i thought maybe it was a fungus of some kind because it started to show on other leaves and was worse on the bottom of the leaf. Kind of soggy feeling

Nah, I really don't think you have a problem. Post some more photos if you have any better views of what you are talking about.