It's not centralized to a single employee. It's institutional knowledge that you don't know. All the coders know how to write code. You don't understand the processes that happen in a lumber mill. Now because lumber coder left, you're maintaining code for a lumber mill. You need to learn that stuff, but you've been working in the concrete division of the company. Get it?

It is when nobody is left that knows something has been documented in the wiki, source repo or google docs.