OK, thanks, I've now added (and run) CheckPlaces, and while it didn't seem to have any luck finding missing favicons, it did find that a couple of bookmarks I haven't used in ages are now dead links.
(Edit, forgot to include in this posting) I'm not clear what their definition of "Questionable" links is, of which it found quite a few. It seems to somehow relate to redirects, so possibly my bookmarks just aren't up to date for those sites. Fix-all for this tab didn't seem to do anything, but maybe it did the corrections quite quickly so I didn't notice ... I'll try another scan later on to see if I still get them.