I have an open bottle of lemon juice that I've been keeping in the fridge. I just took it out to dress a salad and it fizzed like soda when I opened the bottle and smells like... took me a minute to place it... yeast. What's going on? Should I throw it out?
It sounds like fermentation to me. That's what you'd expect when you're making mead or hard apple cider.
I'd personally dispose of it ... partially because I can't think of a good use for alcoholic lemon juice, but also because you don't know exactly what the bug is doing the fermentation.