I have Premiere Pro CS6 - I am running on a Dell T3500 with two monitors. I recently did something to cause my Program Monitor to display full screen on my 2nd monitor. I can't shrink it, move it or even close it unless I click out of the program - then it goes away. It doesn't display that way on any new projects - just the ones that were under way when the problem appeared. I have selected the default editing mode, the 5.5 editing mode, etc. - everything changes but that image on the 2nd screen. I also have my program monitor running normally in my standard layout. Does anyone have a clue what is going on?
What happens if, in one of those faulting Projects, you Reset Workspace?
Go into your Preferences then Playback. Uncheck both of your monitors - this will force Premiere out of monitoring on your second monitor.
If that is left on, in your Sequence Settings, you have a button marked "Playback Settings..." That allows you to set monitoring on a project by project basis, but sometimes it doesn't want to be changed after the fact.