Sorry for being late by 2 weeks here, but Notepad++ would have been exactly what you were looking for, since one of the search features can scan an entire directory for all instances of a string or phrase. I use the feature pretty regularly in updating mods and game files, particularly in maintaining a fork/redux for an otherwise abandoned Stellaris mod with a friend of mine.
Just know you'll need patience depending on how large the directory is and the file types in them; I'm not joking that it searches for ALL instances. It'll chug for a few minutes in its search if you ask it to start looking through a folder with a lot of mid-size images or non-cleartext file formats, but it WILL find your request, even if it then hightlights the specific discovered result (from, say, a DLL) as incomprehensible symbols because it pulled it directly from the binary. That said, you should be able to scan a normal mod directory within 1-2 minutes.