I think what happened in my situation in that my thread started in "All Things Plants". Then because the topic became more of Facebook instead of my own plants or plants in general or gardening, the thread was *moved* to the "Sandbox".
Because I actually scrolled down and saw the thread in the 'Recent Posts' and it was listed as "Sandbox" forum.
So perhaps if a thread starts in a catagory (sp?) thread that is first updated in 'Recent Posts', that it might still show up in 'Recent Posts'? Was I seeing things? I didn't test and see tonight while posting how far down it would let me scroll.