"Guild Vault" tooltip string
 

On an enUS client, hovering the tooltip over the guild bank pops up a standard world object/doodad tooltip that reads "Guild Vault". (I don't mean hovering over items or tabs in the vault, I mean the giant wall safe or totem pole or whatever that you click to access the contents.)

Where is that string coming from? Nothing in GlobalStrings.lua even contains that phrase outside of an informative voucher-related message. The string that I thought would contain it, GUILD_BANK, is set to "Guild Bank" instead.

World entities use what ever the server tells them, that's why players or NPC may appear as "Unknown" when problems occur, because there is no LUA data that contains this information, it's all happening on C level, behind the curtains.

This is why you need to tooltip scan, the game is nice enough to put these special values on the tooltip when hovered, you can do that and scan the tooltip, but that's all you can do really.

only string i found was GUILD_BANK = "Guild Bank";

In the example you mention, "Guild Vault" is actually the name of the three-dimensional world object -- like "Peacebloom" or "Mailbox" or "Portal to Orgrimmar". GlobalStrings.lua doesn't generally include object or NPC names, so that wouldn't be something you should expect to find in there.