I think this problem has been around for me since Mountain Lion, but I can't remember that right now. I updated to Mavericks and suddenly I noticed that I have a menu bar item that has a fixed place but doesn't show anything. I can click on it, and it disappears, only to return when it damn well pleases. I have a dual screen setup, and if I click it away on one monitor, it stays on the other until I click that one too. I've gone through my login items and there's nothing there I think is the culprit. I'm the only user so another accounts login items is no worry. I have a theory though - I've always connected my iMac through WiFi before, but now I've switched to ethernet, and therefore I just CMD-dragged the WiFi icon off the menu bar. I didn't do a clean install of Mavericks, so maybe there's something left from ML, but I switched to ethernet while running ML so that's pretty improbable.