General :: Setuid Process Cannot See Processes Owned By That User

I would like to give a non-root user (nicollet) the ability to detect and send a signal to processes started by Apache2 (those processes are FastCGI scripts and the signal tells them to empty their cache). The processes are owned by the web user (www-data), and I'm running on Debian unstable.

I can't find any way to have the nicollet user see those processes.

The processes are running and can see by both root and www-data:

root@linux-01:~# ps -Af | grep baryton
www-data 17649 17648 0 10:27 ? 00:00:00 baryton
www-data 28145 1 0 Nov01 ? 00:00:12 baryton --bot
root 18701 18700 0 10:46 pts/0 00:00:00 grep baryton
root@linux-01:~#

[Code]....

The most surprising is that the grep process is indeed run by www-data (because it's started from a setuid executable) and is visible, but the baryton process isn't.

What's going on here? Why can ps run by www-data show those processes, but ps run by a setuid executable running as www-data cannot, when it's started by nicollet?