I've seen a number of people mention that there's no real feedback to show when you have hidden object/sub-objects in your scene/model and thought it might be a good idea to mention it here. It's probably most significant when hiding sub-objects and then forgetting you have hidden parts in the model which can lead to a lot of frustration.
Having a hotkey (like a momentary switch) that brings hidden items to the fore and highlights them a user-defined colour while the key is held down then let them vanish again when the key is released could be handy to (1) show you what's hidden or (2) remind you that something is hidden and show what it is. This can help if you open or receive a file where the previous user didn't un-hide something before passing it on.
Furthermore, adding a "statistic" that displays the number of hidden "objects" or "sub-objects" can serve as a constant reminder that you have (or don't have) hidden items in your scene.
