Have you ever recognised one of your friends on this forum and been equally surprised with one another?
Not happened to me but it sounds like one of those things in a film that may happen in real life when posting up pics on the Boy mode vs Girl mode section!
If not how do you thin you'd react? Would you PM them or would you text them on their mobile? Or would you not confront them at all and pretend that you never saw them?
Personally I'd maybe send them a very sensitive PM to them, hoping that it's ok that I know and that they know I CD too, so they should relax.