There is a right way and a wrong way to hide objects in your SketchUp. The more common way to hide objects is to select and right-click to hide them. Fig. 1 this way is problematic, because you must really know what you are doing, or at least remember what you were doing, to undo is not to unhide things that you have selected. This it is easy to forget what you have done. Fig. 2 to make the case even worse, how do you find the hidden objects in the scene? The easy answer is, if they are components and labeled, then you can always find them.
You can unhide by menu all or last, by this may not be the best option. Let's say you need a few parts to be hidden at one time, because you working on a wall or something that you need work closely with. Fig 3 lets say you need room 2 and wall 2 hidden, cause you need add something to the space. After you are done adding the stuff you want, how do you toggle on just room1? As in Fig. 3. The answer is the outliner.
The more pragmatic approach to hide and unhide is to use components with outliner. By making everything in your scene a component, you can control what is visible by a click of the mouse. Fig. 4 with a mere glance at outliner you will know what is selected and visible or not. Fig. 5 you can click on the tagged item and determine if it is hidden are not - no hunting, no guesswork and no wasted time finding things. To find outliner, go to on (mac/windows) menu windows and select outliner.
Paul Stefanik - email@example.com