why does :save not work with -stdin-

[Mohsin]

I usually do search like this:

$ grep  Word *.* | vim -u myvimrc -

$ cat myvimrc

  :autocmd StdinReadPost * :sav! /tmp/x

but when I quit :q, vim always asks me to save the file again,
why is the file marked as modified?

I tried all combinations of flags, but can't get vim to
mark the file as "saved",

any insights appreciated,
mosh.