I think threads are searchable, but they dont stay on the menus forever. Whether the limit for appearing on the menu is by date or by number of entries, i do not know.

For example, an ancient roll call thread is searchable, but wont show up on the menu?

Eta - i see by looking at the bottom of the menu, the search is by date, the last 100 days. Seesaws post is beyond that range.