This is something that for some reason is really difficult to explain written down (my fault not anyone else's!) but it's probably best to explain it like this in relation to the thread I linked above which is a typical example.
This was a thread with lots of posts which was regularly in active convos. I posted on it in the early afternoon of 14 June and then did stuff in RL. I popped back on to Mumsnet just before going to bed, but didn't log on as it was late and I wasn't going to post. This thread and a couple of others I'd remembered, seemed to have disappeared. There was a strange time gap on the Relationships board which looked as though a few threads were 'missing'. There was about an hour time-frame between 21.00 and 22.00 when no-one had posted anything at all. I then did log on and the linked thread came up in 'threads I'm on' and there had been a few more posts during the evening, the last one at 21.51. But according to the Relationships board, no-one had posted anything at 21.51.
My guess is that shortly after this last post, this thread and others disappeared from view to everyone but by the following morning, the thread was back and the first post was at 6.12 in the morning.
Because I was away doing other things, I've got no idea when the thread actually disappeared from the board (or when it re-appeared in the night), but obviously at some point after the last posts on it that evening.
A few days later, the same thing happened again with a gap of 3 hours 20 minutes on the Relationships board where apparently no-one had posted anything at all on that board - a very unlikely occurrence on an average Monday! I recognised at least one thread was missing that I'd seen earlier but hadn't posted on. I remembered the OP's name and did an advanced search and it hadn't been deleted, but had attracted no posts for ages. It did attract further posts when it re-appeared on the boards and was visible to all.
One of the reasons why I don't think it's me hiding and unhiding threads (apart from the fact it's something I've never done) is that presumably you have to log on for Mumsnet to hide the threads you've chosen to hide? I didn't log on when I went looking for that initial thread and it was still hidden from view.