Mysql SHOW OPEN TABLES shows In_use but nothing in SHOW PROCESSLIST I'm looking for advice for a MySQL database used for my development. I have a table is locked, although no process is active.

Here's some context: I am a developer on this MySQL development database without administration rights. All tables are on MyISAM. For the second time in two days I found that all processes accessing a specific table were waiting for a lock. I'm running those 30 concurrent processes on the applicative tier. Those processes are doing steady updates on this table (probably adding like 40 rows per second to this table). This table has a primary key, and one additional index. It has 6 columns including 2 with a blob. At the moment, the table contains 23.7 million rows, and is roughly 51GB in size.

When I found my 30 processes were all stuck due to this lock on this table, I killed them all first. Then I killed the still opened sessions by doing a SHOW PROCESSLIST and issuing KILL for all sessions. The problem is that even after killing those sessions,  the table in question still shows for SHOW OPEN TABLE a IN_USE value of 2, although SHOW PROCESSLIST is empty. 

I can't figure out a reasonable reason why this would happen. There's no process to kill anymore now. I can't repair the table as the REPAIR TABLE waits for the lock. Yesterday night I had the issue, and this morning the locks were gone like magic. Now it's reoccuring again.

Does anybody have a clue on why this could be happening? Any hint for me or my DBA on the right approach to recover and avoid further occurrences of the problem?

EDIT: Strangely, by the time I wrote this, the IN_USE for this table decreased from 2 to 1. And for another table in the same database, moved from 0 to 1. As I'm not doing anything that could have explained the latter increase, I suspect MySQL might be doing something special under the hood.
Speculating, since the tables are in MyISAM, perhaps there is a corruption in the table(s). Since it is locking when checking with the server online, try using myisamchk on the stuck table(s) (and perhaps all the tables) after shutting down the server if possible. 51GB is a lot of data so it may take a while. If possible, suggest following commenter's recommendation to convert the tables to InnoDB; adjust server configuration accordingly.

Ref: http://dev.mysql.com/doc/en/myisamchk.html