/usr/libexec/mysqld: Normal shutdown, but my team doesn't do that?
Linux's Viewpoint
You should run this Linux command
history | grep mysqladmin

This will let you see if anyone ran a shutdown from within the server. Note that this will not let you see remote mysqladmin shutdowns. Perhaps running tcpdump and locating the word mysqladmin or shutdown might help.
MySQL's Viewpoint
Shutdown commands do not exist from command line utilities. Therefore, binary logs will not have shutdowns recorded. You are on the right track when you said;

One possibility I'm thinking is someone (who has SUPER privilege) turned off the binary log then run SHUTDOWN and turned it on again.

Look over the users with that privilege. Run this query:
SELECT user,host FROM mysql.user WHERE shutdown_priv='Y';

You may see something like this:
mysql&gt; select user,host from mysql.user where shutdown_priv='Y';
+-------+--------------+
| user  | host         |
+-------+--------------+
| root  | localhost    |
| root  | 127.0.0.1    |
| root  | ::1          |
| root  | 10.240.163.% |
| root  | 10.48.19.%   |
| user1 | %            |
+-------+--------------+
6 rows in set (0.00 sec)

mysql&gt;

You will see all the users that have shutdown privilege. SUPER privilege does not have the shutdown privilege. There is a separate privilege called SHUTDOWN.
You can quickly revoke that privilege. For example, note the user user1@'%'. It has remote shutdown privileges. You can yank it with
REVOKE SHUTDOWN ON *.* FROM user1.'%';

or
UPDATE mysql.user SET shutdown_priv='N'
WHERE user='user1' AND host='%';
FLUSH PRIVILEGES;

That takes care of the remote shutdown privileges.
You should also address securing user connections by removing SHUTDOWN privilege from every user that does not need to have it. Perhaps you can do this:
UPDATE mysql.user SET shutdown_priv='N'
WHERE CONCAT(user,'@',host) NOT IN
('root@localhost','root@127.0.0.1');
FLUSH PRIVILEGES;

That way, only root@localhost and root@127.0.0.1 can do mysqladmin shutdown within the localhost. I would definitely have the root@127.0.0.1 on hand because there are times when mysqld loses connection to the mysql.sock file and prevents root@localhost from connecting to issue a shutdown. With the root@127.0.0.1, you can issue this:
mysqladmin -uroot -p -h127.0.0.1 --protocol=tcp

to issue a mysqladmin shutdown without needed the socket file.
Give it a Try !!!