In a recent WTH moment our own Heather Alley realized it is the little things in Revit that will trip you up. As you will see here she ran into an issue with Revit where you cannot select a filled region and after choosing to Edit Boundary the sketch lines do not show up?
Quite the mystery!
You can see the hatch fill pattern, but you cannot pick on the hatch or get to EDIT BOUNDARY.
Answer: The answer lies in VV (or VG) Visibility & Graphics. Open your Visibility/Graphics dialog box, and left click on the little plus symbol next to Lines to expand out its subcategories.
One of your line subcategories is checked off. In the image, mine happens to be "THIN LINES".
Check the box next to the line that is off, and Voila! The hatch fill pattern boundary will reappear.
If the Line type is turned off, it turns it off as a boundary line in the hatch pattern as well. Hope this helps!