Maybe it was a customized add-on (2nd screenshot), put there by another application or the user, and now that application is no longer on the system, or just broken, the "?" appears. Maybe hovering over the "?" icon will reveal something.
Since I'm using OS X 10.4.11, I looked at screen shots at Apple.com and it appears that with OS X 10.6 that there isn't normally anything in that position. My guess is that it was either something that was installed on your Mac (application/utility) that you've since removed or something that was accidentally placed there and is now now longer on your Mac.
Normally if you have something in the toolbar, sidebar or the dock and it's deleted from it's actual position on your Mac you'll get a question mark? If you hover over the question mark you should get a tool tip that says what it used to be. You can get rid of the the question mark by Command clicking on it and dragging it off the toolbar, it'll disappear in a poof of smoke. If you want whatever it was back you'll have to add it again by finding it on your Mac and dragging it and holding over the toolbar until a space for it appears and then let go of it. It will then be in the toolbar as well as the original location on your Mac.