I actually cleared the place out earlier and then one of my settlers gave me the mission to clear out the Wicked Shipping lockup... but as the ghouls were all dead I cannot complete this mission... even though it is done.

EDIT: Solved it... a Ghoul was actually phased into a box and had to shoot the bit that was just visible and he then crawled out through the side of the box lol.

Preston Garvey assigned me the quest "Ghoul Problem at Sanctuary Hills". I've had a similar quest before that worked just fine. A settler asks me to help clear out the ghouls at the Super Duper Mart, so the first portion of the quest triggers properly. Having been to the Super Duper Mart before, and having previously cleared out all the Ghouls that spawned there, I traveled again. However, the marker that shows the quest location does not show up when you push X on the PipBoy, it just shows you wherever you're standing. When I got there, I never got a ghoul to spawn that had a quest marker over his head. I killed everything inside the building anyways, hoping it would trigger the completion of "clearing out the Ghouls", but nothing happens. I've gone back to the location on several occasions throughout several in-game and real-life days, brought several different companions, and I cannot trigger this as completed. Playing on the XBOX, I can't force the quest to fail, and it remains open always. I assume that because the quest triggered properly at the beginning, it won't fail out due to time, since I've already started it. I'm trying to increase the settlement's happiness to 100%, but now Preston/other settlers have stopped assigning me quests.

Did you have the box that shows the quest location at least when you were able to complete this? Anyone else who could help this would be greatly appreciated!