Filter on keyword across multiple "folders"

It is clear that I can filter for specified keyword within a "folder."
This does not provide much convenience, it seems to me. I want to locate images with a given keyword across a large number of "folders" including even my entire C: drive. I must be missing something, because there is little advantage if filtering for a specified keyword is limited to what you might see in the thumbnails on the Bridge screen.

I nearly always store my images by date, with the "folder" name being the date taken. I plan to use Bridge to assign multiple keywords to all my images stored on my computer, a tedious chore, but one that I can do a bit at one time; say one day's shooting per day until I eventually catch up with several years of shooting.

There must be some functionality that I am ignorant of which allows the user to filter on all, repeat all, images tagged, for example, "Sarah." I have tried searching the FAQs and have failed to find the answer. I have tried highlighting a couple of "folders" but the "filter" functionality is not available as far as I can tell. Please tell me what I must do to exploit the Filter functionality across multiple "folders."