The problem isn't with the search term exactly but with that combined with the location of the search. Spotlight doesn't find cache folders if the search location is the entire computer.
A more specific search location will find what you are looking for. Try this:
1) Enter Find mode with Cmd-f.
2) Click the "Others..." location button.
3) In the sheet that drops down, add /System & /Library to the list by using the "+" button or dragging these folders into the list from a Finder window view. (They will remain in the list until you remove them, so you only need to do this once.)
4) Click their boxes to make them your "Others" search locations & then click OK. The search location button changes to "2 Places."
5) Enter "Caches" (no quotes) in Find's search box.
Ta da! The caches folders should now be in the results list. (If you add your home folder to the list as in steps 3 & 4, Spotlight will now find the user cache folder in ~/Library too.)
Note that if you change the location to "Computer" the folders are no longer found. Switch back to 'xx Places' & there they are again.
From this, you might conclude that a "feature" of Spotlight is to exclude cache folders from searches on the computer location. Nope. It will find specific caches by partial name matches: try "DataCache" for example & you should get a few hits in /System/Library. Try "Cache" (the singular instead of plural form) & you do
not get the Caches folders but you do get a long list of cache files in them.
BTW, there is a minor privacy concern with this. By default, Safari caches web site 'mini' icons for use in bookmarks & the address field & the cache search finds them ... for all user accounts, not just the active one. By examining them, you can sometimes see the web site & date it was visited by other users.