Further explanation: It is not the dock I am refering to and efinitely, not an app. Apps don't have these symbol in their toolbars. So perhaps it's not called a toolbar. For example, If I open the Mac harddrive window, I find the following items at the top (toolbar). Going left to right; forward and back arrows, red, yellow and green buttons, the question mark, four boxes labeled View, three boxes labeled Quick Look, Drop Box, and Action, Search window, a folder labeled Miscellanous Docs, and finally, a minimize button.
Yep, that's the Finder toolbar.
You can put anything you want there, though it's not obvious and a lot of folks don't realize it. You put things there by clicking on an icon and dragging it to that part of the Finder window. I've seen many people do this by accident when they try to, for instance, move a file from one folder to another.
The question mark means you put something in the Finder toolbar, then later deleted or moved it.