The Scene instance can be found from View → Scene instance. Here you can see all the instances, objects, entities etc, you have inside your level at the moment. Most of them will be under a DummyNodeEntity, but by clicking the + sign you can open up and see the actual objects inside the level. Be prepared to see many objects with identical names if you haven't been naming them all separately. You can quickly open this one up with Ctrl+F2.
Just like most other windows, this also contains a search function and filtering options for finding specific objects faster than going through the whole list manually.