On two instances, I've been unable to empty my trash because it contained a pdf that the system claimed was in use. On both occasions, I typed the lsof command to find out which app was using it. In both instances, Preview was indicated — yet it wasn't shown in the Dock! Activity Monitor does show Preview running.
How can this be?
Best Answer
It's possible that the plist of the application has been changed (eg, /Applications/Preview.app/Contents/Info.plist). There is a setting LSUIElement, which if is true, causes the app to be hidden from the dock, the ⌘-Tab application switcher, etc. See this similar question for more details.