One thing about Aperture that's really cool is that any search criteria active for any project, folder, album, or other image containing entity is retained independently of each other. All in all this is helpful for switching back and forth between multiple image views with a unique set of image display criteria. Even if you exit Aperture 3 and relaunch all of the various search criteria will continue to be intact for every single project, folder, album, etc.
If you select three star and above images, with a flag, a blue label, shot with your Nikon D600 using a 50mm lens at f/1.4 for all of the images in a particular project then exit Aperture and don't return for that project again for another year that search will still be retained and active in whatever project you were looking at it the next time return to it. I present this as a feature with merit because it does have merit and obviously intentional. If you are new to Aperture this might throw you off a bit. Here's a practical tip.
If you ever click on anything in the project inspector and see no images or something different than you expect in browser mode take a look up at the little image search box. Is there an active search that you forgot about? Click the little "x" to reset the criteria back to unrated and above. It might be a good idea to check all the time. Get in the habit so you are looking at the images you expect to be looking at.