Can a USB outlet be causing a Flat Battery ?

"water got in". Replace them both or dry Thoroughly. Keep them dry and/or seal with something. Even 2 USB chargers are a minimal load without charging a device. If you simply dry them, use a Milliammeter in series with the hot lead, no load; it should show less than 200 mA. If more , one or both is shot. .These are relatively cheap; a new Yuasa battery for most 'bikes is $120 and WON"T solve the problem.

You might also want to see if your are getting over 12,6 volts at 2500 RPM. Should go to 13.6, min, with lights on.

My cell charger is plugged into a cig liter socket I added, when I don't use it, I pull it out enough that the pilot LED goes out,. No drain. I also have a reverse protection diode and fuse on it.

There's some damage you haven't seen yet from the water.

Couple of things, if the USB is connected to a computer, then it won't be charging when the computer isn't on, and will probably leak a small amount from the battery. If it is plugged into the wall, that shouldn't happen if the power brick it functioning properly. Of course they can fail, and not be charging correctly. They get hot, and heat destroys electronics over time, so it's not impossible for one to fail just enough to work just well enough to function poorly.

The other thing, though, is that having any battery, connected for very long periods of time without some device or software to regulate the charge is going to cause the battery to fail sooner. Batteries have a set number of charge cycles, and something that keeps adding little bits of charge when it drains a little bit naturally uses up some of those cycles. It isn't much, but over time you are talking about premature battery failure. A regulator will allow the battery to drop a certain amount before applying a charge so it spends less time in charging mode. Laptops do this with software/hardware.

Lot's o' stuff online 'bout how to install them accessorizationals, like with a switched relay, dude. There's a reason fur dat. Think about it. Sorry about the thinking. https://www.youtube.com/watch?v=FTO5KfIyaO8

Just watch porn vids