Ok, so the repair shop said they would solder the drain area.
But, there is a plastic piece inside the tank that has fluid in it. (We rinsed the tank with water after removal.)
Anyone know what this piece is? It looks like it is just an air chamber?
The shop wants to put a tiny hole in the bottom to drain.
They are going to fill the tank with inert gas to solder the problem.

Could it be a float from the fuel level sender that became detached?
They are about the size of a "D" cell flashlight battery with a ring-shaped
depression around one end that snaps into a wire arm.
--
George Downs, Bartlesville, Heart of the USA!

No, I saw the float on the fuel level sender when we removed it from the tank.
This looks like a white plastic box, much larger than the float, at the top of the tank.

If it could enter the tank by getting in there via the filler neck - why not just think of the item as a fun thing happened to your car down the road?

If it aint the sender-unit's (or a previous broken sender unit's) - it's just something that someone threw in there for fun or the like... :-)

It's an UFO (Unidentyfied Floating Object) - and has no purpose in your tank.