Blowby
I had a similar problem on a Rabbit i used to own. Go ahead and laugh but that thing could go all day on a dollars worth of gas.
It was a clogged Catalytic converter.
I had been hearing a noise for a month or so that i couldn't figure out. kind of like something rolling around under the car. The it started blowing oil out everywhere, and i mean everywhere... dipstick, valve cover, pvc valve, everywhere.
A friend told me to drop the exhaust before the cat and try and run it. It ran great.
So what ended up happening was the inside part of the cat, had come away from the sides and wore itself into a ball and was rolling around it side the housing... and that was the sound i was hearing. And it had finally wedged itself blocking the exhaust.
Put a new cat on cleaned up the motor tightened a few things down and drove it another 130 thousand miles.
so if one or both of your cats a clogged you can get the gas exhaust out and it has to go somewhere. it will take the path of least resistance.
Tony