I think I've figured this out (and my apologies if I should really have directed this to moodle forums).

Going straight to the database and looking at the mdl_log table, all entries for guest user (userid=0) are for login errors on frontpage. This seems perfectly logical.

The odd thing is that the statistics page includes these entries, but they are omitted from the log report, even though Guest User is available in the list of users to filter by.

It's not a major thing, just had us a little worried for a while.