This leads me to believe that the
mysql package was updated and
mysqld was not restarted. The old binary was deleted and a new binary installed.
In Linux, deleting (unlinking) an open file does not really delete the contents of the file. A processes that held the file open before the unlink will still be able to access the file until it exits. When all processes close the unlinked file, the contents of the file become unreachable.
In this case,
mysqld from the old version of the package is still running and
/usr/sbin/mysqld is a link to the new version of the file.
Fun fact: You can actually get the original file back using
cp /proc/2589/exe /tmp/mysqld