ubuntu登录进去的时候老提示"There is 1 zombie process"，但实际没找到相关进程

我经常从ssh登录到我的ubuntu服务器时都会显示

Welcome to Ubuntu 11.04 (GNU/Linux 2.6.38-16-server x86_64)

• Documentation: http://www.ubuntu.com/server/doc

System information as of Wed Oct 17 22:07:28 HKT 2012

System load: 0.16 Processes: 144
Usage of /home: 48.8% of 18.33GB Users logged in: 0
Memory usage: 1% IP address for eth0: 192.168.1.201
Swap usage: 0%

=> There is 1 zombie process.

Graph this data and manage this system at https://landscape.canonical.com/
但我用下面的命令

ps aux | grep Z
却没有找到任何STAT为Z的进程，请问这个提示是什么意思？我怎么才能找到这个进程呢？