This is a fix if you (like me) were on the Oh My Papa quest and needed to convince Melissa to break the alliance with Caesar, only to find her missing from the Great Khan encampment and no where to be found.

http://fallout.wikia.com/wiki/Oh_My_Papa#Bugs

Here's the solution I used, sadly this only works for the PC, if you're on a console you're [censored] and I'm really sorry

Fast travel to the Great Khan Encampment, open the console by pressing the tilde key (or whatever key you use to open the console, on mine it's |) type in exactly the following:

prid 000e5e51

Then press enter, open the console again and type the following:

enable

This should make Melissa appear in your vicinity and approach you.

The issue was simply that the game disables her after you complete the Sloan mission, she's actually still there, which is why your quest marker leads there she's just not visible or interactable.

Tags for google:

Melissa Great Khan Great Khans Fallout New Vegas bug where is Melissa

Note that 000e595c is the BaseID.