I have gotten this icon stuck on top of the left sidebar (I circled it with red, just below the menu and there doesn't seem to be a way to remove it. When I right click on the icons below, it's the Task Manager and there's a close icon but when I right click this one I get Icon settings and Panel settings and nothing more.
The end of the panel has a little doohickey like yellow yinyang symbol called the Panel Tool Box. You can also right click an empty space and under Panel Options find Panel Settings. It is not confusing to find Settings under Options neither it is that the doohickey popup calls it Toolbox while this menu calls it Settings. Now you if you hover the cursor over that thing it'll turn into a four arrow cursor (because you can move it) and then you can right click and remove it. It's not hidden at all. This is documented at http://userbase.kde.org/Plasma/HowTo/ but good luck figuring it out.