Of the three rain barrels we have, the one I made from a garbage can only has about two gallons of water in it. The other two are fully enclosed, plastic, 50 gallon barrels, and they are full to overflowing.
The one I made is a 30 gallon galvanized garbage can, and I had removed the lid yesterday, since the lid blows off easily enough. When I was just out with the dog, I saw only about two gallons and a bunch of leaves in the bottom of it.
I watched as the water dripped out of the downspout into the barrel. And I didn't see any leaks. The other day after a rain, it filled and held water.
So, it's getting water, not leaking, yet not filling.
My theory: the wind is strong enough over the top of it to literally suck some of the water out. I'll admit, farfetched.
I'll take your theories in the comment section.