I’ve heard some people use it when they don’t have the rising inflection in their voice that usually denotes a question. I’ve also used and seen it used to mean dubious claim as the other poster mentioned! Not very commonly though.

I use it as a way to say I’m pretty sure the thing I said is true but I have doubt.

" the glasses are only cosmetic, question mark"

Also turns out I’m pretty sure the devs patched the glasses,and they now negatively effect aim.