Are there places and items in-game that no PC has ever seen before? I guess this could apply anywhere, but I'm specifically thinking about the top-tier secret places like hell or silent tower. I would imagine there's some even non-limited items there no one has ever held. Are there counters on things to determine how frequently and/or if they've ever been had?

This aspect of the game has always been interesting to me. It would be interesting to see something no one else had seen. But how would you know? It would be kinda neat if a skill like attune could give you that sort of information too.

... none ever figured out all of ST before it closed.

In hell some imm said that like 50% of the area haven't been explored. heh

Always shoot first and then call whatever you hit the target.