How can I find the Unix process that owns a local Sleeping MySQL connection?
If you can, change the way you are connecting to the mysql server by using 127.0.0.1 instead of localhost.
By doing that, a tcp/ip connection will be used and you will have in the processlist the port number that you can use to trace back to the process.
A tcp/ip connection is slower than a unix file socket connection, but it'll do the trick.
references :
http://dev.mysql.com/doc/refman/5.1/en/can-not-connect-to-server.html