When the last copy of a source is removed from a scene, the source will be removed from OBS Studio and thus disappear from the Add Existing list. Why would you store unused sources? As mentioned above: the visibility can be toggled off by using the eye icon.
If you really want to store unused sources, maybe it is a workaround to create a scene and call it i.e. Storage. Add your scenes there before removing the last one from your normal scenes.