That's true if the right side is a flue, but with neither fire brick nor flue liner in evidence, I suspect it's purely decorative -- I'm interpreting OP's text description to mean that we're looking at one of two lined flues, and one of two unexpected void spaces he found when removing the cap. I don't think the void is actually connected to anything, and instead is just decorative, but if I'm wrong and you're correct, then yeah -- there are more serious issues to deal with here