By default sys-apps/sed installs the binary in /bin, but a lot of programs search it in /usr/bin (es. eselect, run "grep sed /usr/share/eselect//libs/core.bash" to see this). In fact,"equery b /usr/bin/sed" and "qfile /usr/bin/sed" don't return any matching package, so this symlink doesn't belong to any package. I think that is a "forced" symlink, inclueded in stage3. If my suppositions are true, i suggest to add a /usr/bin/sed symlink creation on ebuilds.
*** This bug has been marked as a duplicate of bug 122260 ***