I don't know how he ended up there, but I have a suspicion. When that mission type triggers, it is supposed to select a building along their route which is not the home base and not the scavenging location. The problem may have been caused by it not being able to locate an appropriate building and doing a fallback selection of a random location. There is a delay before they head out though, so unless you do have a lot of patience, it is best to just call them and move on. (Also, like any NPCs, when you are near them they become vulnerable, and can be killed by zombies.) However, the mission indicates for them to continue to the target (or back home if it happens after they have scavenged), so they won't get in the car. Overall, I don't think it is a bug as much as one of the rare circumstances, in this case not having any intervening buildings between the home and target. I am surprised the mission triggered that way at all, as that mission type has a low chance to happen, but is checked periodically, so the farther they have to go the higher chance of it triggering. With such a short distance, it is quite unlikely. One other thing that I think you know based on what you mentioned, but to clarify... after you call for the scavengers, you can take the last resource and they will still find another one upon arrival, so you don't have to wait for them to get there. What they will find is set as soon as you call them. But... if you log out before they get home, you will only get some morale and influence, not the resources. For some reason the game doesn't save whatever resource type they are carrying, and so if they are out when you log off, when you log back in they don't have a valid resource type... so you get only the generic rewards and none of the specifics.