I don't use any Twitter plugins and this cookie only appears if I enable the Twitter sharing option in wpForo. Are you sure you have Twitter sharing enabled?
It looks like you have filtered cookies for wpforo.com. But external cookies are stored under their corresponding domain (but still, when they are set on your site you are responsible for them). Instead, click on the little "i" in the top left corner of your browser. There you should find the Twitter cookie.
This cookie appears as soon as I enable Twitter sharing in the wpForo settings.
Indeed, it seems to be empty. I didn't know that was possible. So what exactly is this if it's not an actual cookie?
@robert So what exactly is this if it's not a cookie? Because it still only appears when I activate the Twitter sharing option.