That's a really good question. The search function on site doesn't work, as we know.
I tried hiding a thread, having first memorised a significant phrase in one of the posts, and then googling the phrase, preceded by "Gransnet conversation" which usually works, but came up with nothing.
If you have hidden, say, a games thread, and then look at the games forum, it is hidden there as well. That is, after all, the point of hiding it.
You could try logging out and looking for it. If you are not logged in, there will be no record of your having hidden the discussion. If you wish to contribute, however, I'm not sure if you then can.