That makes sense now. Like I said, the class is new to me so be gentle. :-)
Luckily enough, your Faction Mission works perfectly with the retconned back story I proposed so that'll be it - you were providing security for another Society expedition that involved crossing the island by foot with a small team and ending at this expedition's camp. You will have just arrived in the camp when the rest of the group returns with the bodies and the wounded.
Feel free to read through the adventure log so far and thanks for saving my bacon.