Run through this checklist should help you find those objects.
3. Filters – MEP mostly use these to differentiate the difference between systems, but can be overlooked regarding visibility, make sure to check the filters are visible otherwise the objects will not display
4. Worksets – Control visibility of objects, just off or on
5. Revit Links – for the linked disciplines, they can be set to match the current view settings OR the settings from a linked file OR to custom
6. Discipline – Can affect the way objects are displayed ie in a Mechanical discipline the arch is hardcoded to be halftone
7. Hidden Objects – Objects can be turned off in a view, using the sunglasses icon on the view control bar, use the light bulb next to this to check for temporarily hidden objects
8. Crop Region – Turn off the crop region and display it, are the objects outside the crop?
9. Import Categories – When creating a family using a imported CAD file for an annotation symbol you should explode the symbol, otherwise you can turn off its display and it will disappear