Yes i get it. The input currently at the top is FILTER, which means it filters from current loaded items. This is fast, effective and happens in real-time in the browser via Javascript. What you are asking for, is SEARCH, which needs to be done on server (PHP) and will be much slower of course, proportional with the amount of files and folders in your entire folder structure. This has been requested before, and I hope to look into it at some point. I just want to clarify, FILTER and SEARCH (deep search) are two completely different features, at least from a technical perspective.
A global search feature wouldn't just be a "folder finder", but any file finder.
The reason I mentioned a filter option above the menu, is that this would provide a super-fast way to search all folders in the menu, similar to how filter works for current files.