I'm having ST and a browser open next to each other. When I hide the side bar, and move from ST to the browser and back, the side bar reappears.
OS: Linux Mint 12
I'm having the same problem. Very annoying. Hide sidebar, edit, switch to browser and back, and the sidebar reappears, hide it again ... ad infinitem!
You may have installed syncedsidebar plugin. Don't blame the author, can't know if the sidebar is open or closed. Sublime limitation.