So the other day two of my colleagues came to me because they had a sheet that they could not find in the project browser. We knew it was an actual sheet not a dummy sheet since we set it up and put views on it. It was not deleted because it showed up in the sheet list and we were not able to renumber a sheet to it's number.
This my not be a large issue to some, if you primarily go into the views to make changes rather than work of sheets, since the missing sheets still reference the same and print the same as well. If you need to get to the sheet to make changes, where did it go?
Ben May of Revit-Ninja pointed out a case where his sheet had been moved up with views in the project browser. This happened to be the case for me as well. So to find this sheet, right click on "View (Plans/Details)" in the project browser, and turn off any filters and folders. You should now just have a long list of all your views. Scroll down to where "Sheets" would be alphabetically (ie "Sheets: RF-102S) and low and behold there is your missing sheet.
That is the solution if you don't mind switching up your browser to find that sheet every time...if this is an issue you can always delete the sheet in the Sheet List (right click and delete row). This will release all the views from the sheet so they can be replaced on a new sheet of the same name and number. Granted this may not always be very friendly if you happen to have 30 or so details on this sheet.
Now, AutoDesk says this is a very rare occurrence, but there is no way to fix this instance of the sheet to get sheets listed back where it is supposed to under "Sheets."