> grep is missing a hook similar to occur-hook. It is very useful to know
> when results are returned and also to perform operations on the resulting
> buffer.

Does this work for you?

#+begin_src emacs-lisp
(add-hook 'compilation-finish-functions
          (lambda (cur-buffer msg)
            (when (and (string-match-p "finished" msg)
                       (eq (with-current-buffer cur-buffer major-mode)
                           'grep-mode)) ;; i.e. not compilation-mode
              (message "Grep finished in %s" cur-buffer))))
#+end_src