In InDesign CS6, the weirdest thing happens: Whenever I open InDesign, it automatically opens the last document, even though I had closed that document before quitting InDesign previously (in other words, there are no documents open when I quit InDesign). Maybe this is supposed to be a feature, but personally I find it very annoying.
Thanks, that indeed was the problem. But oddly the default seems to have switched from Lion to Mountain Lion. Instead of having to uncheck "Restore windows when quitting and re-opening apps", now I have to check "Close windows when quitting an application."