External monitor, iMac, detect when turned off

I have an external monitor connected with DVI cable and through the mini-display port. For the first two months that I was using it, my iMac would detect when I turned on the second monitor and allow for a second screen. And then when I turned the second monitor off, my iMac would detect that and go back to using just its screen.

Now, however, it does not detect the external monitor has been turn off and I can lose windows on my turned off screen, which is really annoying. I have looked a lot in display preferences but found no way for the iMac to automatically detect that the second monitor has been turned off. Even when I click "detect displays", it still detects the second display even though that monitor is turned off. If I unplug the cable from the mini-display port and then click "detect displays", it detects that there is no second monitor. Any ideas about how I can get things back to how they were when I started with my iMac and why it is that this changed on its own?

Thanks.