While its something none of us would lke to think about, it does sometimes happen that very old and infirm dogs take themselves off to die somewhere away from their owners.
This is what would happen in the wild, and even when I lost mine old girl on Christmas night she got right under my bed where she never went and curled up and went to sleep, when I felt her she had died.
I do sincerely hope that if this has happened you find her somewhere, so you at least know she went peacefully, and
can have closure.
However, there is still hope and the doglost volunteers will not give up until there is some definite news.