Since you can add to `org-id-locations-file' without having to put the files in `org-id-locations' I think it would make more sense to also scan `org-id-locations-file'.
Currently you can empty `org-id-locations-file' (".org-id-locations"):
1. $ emacs -Q <RET>
2. C-x C-f ztest.org <RET>
3. C-RET heading <RET>
4. M-x org-id-get-create <RET>
5. C-x C-s
6. C-x C-c
7. $ emacs -Q <RET>
8. C-x C-f ztest.org <RET>
9. M-x org-id-update-id-locations <RET>
10. C-x C-c
