Even thou I have completed some of this type of missions, I am - like Panopticon - a bit unsure about the condition for their completion.
The flowing I haven't reported on Mantis because I don't know the ratio 'bug' / 'me don't understand game mechanics'.
Anyway, the other day I went on a secret rescue NPC mission inside a house. I found the NPC and led him back to the mission entrance, then I went thru the entrance so that I was back in the mission staging area. I was expecting to get the message that I had rescued the NPC, but I got none. The mission was still active so I went in again only to find that a new
mission room had been generated, so I started to make my way thru this new room (which had monster spawners in it), after having traveled some distance in this new room I get the message that the NPC is dead and after a few more steps I come across the NPCs vengeful ghost
This has raised a few questions / ideas for improvement. Firstly, it seems possible to (without warning) leave a rescue mission and i.e. accidently leaving the NPC behind. I think it would be great to get a message similar to 'X have not kept up with you. Do you want to leave X behind?', if you are about to leave the mission without the NPC. Secondly, If you leave the mission and thus leaving the NPC behind, the mission should probably be forfeit.