Oh, sorry, I think you're right. I don't see it either. I think I was just remembering it being there. I don't generally pay that much attention to the sidebar :-).
I doubt it's conditional visibility. Why would anyone do that? (FWIW I don't currently see the all-time top 15 but do see the 30-day top 15; I am in the 30-day top 15 but am about 99% sure I'm a long way from being in the all-time top 15.)
Apologies if I misled anyone.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.