What's the deal with hidden items in FRLG that don't appear initially?

Question

Not much to explain here. But in mt. moon and at least the NS underground path there are hidden items that do not appear until a certain point. For the mt. moon ones I think it's the S.S. Anne and for underground path after the rocket game corner. I wasn't super extensive when I tested but the items did not appear there on my initial go through of those areas. I was hoping there was some kind of guide to it.

Answer 1

According to this page, the items in the Underground Path on Routes 5-6 do not appear until you have visited the S.S. Anne. However, this page claims that all but one of the items in the Underground Path on Routes 5-6 do not appear until you obtain the Vs. Seeker from the Pokémon Center in Vermillion City.

Both pages agree that all but one of the items in the Underground Path on Routes 7-8 don't appear until you clear the Rocket Warehouse.

I would try what each page suggests and then see if the items appear.

Neither site mentions anything about items in Mt. Moon not appearing, so they should be there. You might've collected them already or you could be looking in the wrong locations.