diff --git a/site-lisp/db-projects.el b/site-lisp/db-projects.el
index 0ae11fd..a0a0bc1 100644
--- a/site-lisp/db-projects.el
+++ b/site-lisp/db-projects.el
@@ -2,7 +2,6 @@
 
 ;;; Commentary:
 
-;; XXX: update org-agenda-text-search-extra-files when creating and removing projects
 
 ;;; Code:
 
@@ -116,6 +115,28 @@ project diary, and updating projectile's cache."
    (expand-file-name short-name
                      projects-main-project-directory)))
 
+(defun projects--org-files ()
+  "Return all Org Mode files in all known projects, recursively."
+  (mapcan #'(lambda (dir)
+              (directory-files-recursively (expand-file-name dir projects-main-project-directory)
+                                           ".*\\.org" nil))
+          (projects-existing-projects)))
+
+(defvar org-agenda-text-search-extra-files) ; to keep the byte-compiler happy
+
+(defun projects-find-unsearched-org-files ()
+  "Find Org Mode files in known projects that are not searched by default.
+This is done by checking all org Mode files in every project
+whether it is included in `org-agenda-text-search-extra-files'."
+  (require 'org)
+  (let ((extra-files (make-hash-table :test #'equal)))
+    (mapc #'(lambda (entry)
+              (when (stringp entry)
+                (puthash (file-truename entry) t extra-files)))
+          org-agenda-text-search-extra-files)
+    (cl-remove-if #'(lambda (org-file)
+                      (gethash (file-truename org-file) extra-files nil))
+                  (projects--org-files))))
 
 (provide 'db-projects)