I had a Chase MileagePlus Explorer card, which I cancelled due to numerous issues with United. I called up and told them I wanted to cancel the card. They confirmed, and that was that.
However, I can still log on to the website. I can still see that I have a "$0.00 balance". Did they really cancel (i.e. do they have a legal obligation to, and one they usually follow), or should I double-check?