If you've made a symbol and you delete it from a page for later use, it should not be removed from the module library if it is not on any pages. i.e. If you create a Symbol Element (right click, create symbol) it should stay in the module library regardless if it's on any pages or not (make an "Unused" section in the symbols list while you're at it)
Unused symbols remain in the Symbols library for your use. We do denote that it has 0 instances on the canvas though. If you're running into any issues where this is not happening, please contact email@example.com.