Had something similar happen to me one time. Doing a charter Moresby/Mt Hagen/Moresby in a Simbu 402 and upon landing at Mt Hagen was told by the charterer that 'there is an extra passenger' and could we leave immediately!
"Ok, no worries" said I.
The Charterer promptly ushered out his passengers which included a woman who was quite obviously very apprehensive, indeed frightened!
As I went through the walk around after loading the passengers I was was urged by several of them to 'please hurry' and 'we must go now.'
Even as I taxied out I noticed that several of the passengers appeared somewhat agitated and the woman was by now extremely edgy.
And as the wheels left the ground I could have sworn that I heard a huge sigh of relief from all on board.
Shortly afterwards, I happened to glance at the tarmac outside the terminal buildings and noticed a solitary figure running about.
I asked no questions, and no explanation was given.