The Explorer has a Search and Filter feature to make things easier to find. This feature is extremely useful when you need to find objects in a large scene.
Typing into the Search Box creates a list nodes with matching names in real time.
Alternatively, objects can be filtered by type. You can choose between PolyMesh, Light, Camera, Null, Bone, and Material. For example, click on the PolyMesh icon to show only the PolyMesh objects in your Explorer. The same concept applies to the rest.
Hit the X sign at the end of the search box to clear the filter. Missing objects will no longer be hidden.