We get this call all the time. A Revit user has a view that is showing some of their section marks but not all. Revealing hidden elements reveals no hidden section marks. Other views show the same mysteriously absent section marks, and of course, the section views exist in the Project Browser.
This is because of a frequently overlooked unique property of section views. When you create a section, the scale of the view in which you create the section initially sets the default scale beyond which the section mark will be hidden. In other words, if you create a section in a plan whose scale is, say 1/2"=1'-0" you won't see it in any views whose scale is 1/4"=1'-0", 1/8"=1'-0", etc.
To fix it, either select the section mark in a view that you can see it in, or navigate to the section view itself and in the Properties palette change the "Hide at scales courser than" value to a smaller scale. I usually go with 1"=400' myself - I can always hide a section mark "By Element" or "By Category" if I don't want to see them in a view.