Here are all the Expeditions that offer a chance on a new recruit and the possible outcomes they have.

Most events are split in multiple variants which are randomly chosen, some more and some less likely weighted to be chosen by the game. Not all of the listed encounters below are seen often, and if you finally get one of these, there is always a chance to come out empty handed in every one of these.

There is a lot of speculation if the game uses deeper mechanics to determine your success chances, from a seed system instead of a traditional RNG system, to diminishing returns depending on other conditions like how many survivors you already have etc.
I can not find any proof for this in the game files, but I can't do a full code review on a game either, I am just a human with limited time. If anyone asks and manages to get a response by a developer on any of the related questions, let me know :)


There is a flag that an event can be rolled only one time, but I can't tell if this is literally only once per playthrough or if it means only once per in-game year or something like that.

• "Campfire" - Small chance to get a new recruit right away / to find the remains of a campfire / find a dead body surrounded by critters. In the campfire event, when the player shouts there is a chance each to get a new recruit / find nothing / being attacked by a critter.
  
  
• "Rescue Pod" - Chance to get a new recruit / find a dead body / find a dead body surrounded by critters.
  
  
• "Signal rocket" - Slim chance to get a new recruit, or to find a survivor under attack or to find a dead body.
  
  
• "Smoke signals" - Good chance to get a new recruit. Possible to find the remains of a campfire. In the campfire event, when the player shouts there is a chance to either get a new recruit / find nothing / being attacked by a critter.
  
  
• "Thick smoke" - Small chance to find a bloody human at a bonfire. In the bonfire event, when the player talks to the person, there is a chance each to either get a new recruit / get attacked / person running away.
  
  
• "Thin smoke" - Small chance to get a new recruit right away. Possible to find the remains of a campfire or to find a burned bush. In the campfire event, when the player shouts there is a chance each to either get a new recruit / find nothing / being attacked by a critter.
  
  
• "Dark cave", "Large cave", "Small cave" - Can each only be found once. Dark and Large cave: Small chance to find a lair (=when you find a creature in the cave). When the player investigates, there is a chance to either encounter a beast or a survivor that can be recruited.
  
  
• "Security signal" / "Crash site" - Security Signal can only be found once.
  In case of the "Crash Site" event: If the Crash Site "Armory" is rolled, and if the player chooses to look for survivors, there is a chance to find a survivor, a chance to find no survivors and a chance to receive an injury.
  In case of "Security signal": One of three possible different crash sites can be found in this event.
  In case of the generic event "Crash Site" there are 14 possible outcomes, so the chance to find an armory crash site is much slimmer.
  
  
• "Crash site": "Control Module" - 1 out of 14 chance of varying degrees to roll this crash site. If the player chooses to look for survivors, there is a small chance to encounter a survivor that can be recruited. The other outcomes are: Receive an injury, find no survivors or find resources to scavenge.
  
  
• "Crash site": "Cruise Ship" - Can only be found once. 1 out of 14 chance of varying degrees to roll this crash site. If the player chooses to look for survivors, there is a chance to encounter a survivor that can be recruited. The other outcomes are: Receive an injury, find no survivors or find resources to scavenge.
  
  
• "A house?!" - Can only be found once. Slim chance to find a house with a survivor in it, which can be recruited. The other outcomes can be "Nothing found", a house with some old bones and a house that is abandoned.
  
  
• "Landing pod debris" - When you hear noises at a scavenging site and get a choice if you want to jump the creature or peek out of cover, there are two possible human encounters: When jumping out, there is a chance to find a human, but the survivor will be spooked and flee from your explorer with no chance to recruit. When peeking, there is a chance to encounter a human and to recruit them.
  
  
  
  
• "Dune House" - I see a chance in the game files to recruit someone when this site is found on an expedition, but I do not see this site in the possible outcomes when you explore. I reckon this is for an upcoming map variant that is more desert-ish, because we don't really see dunes on the current map.

The Signal Rockets and the Smoke Signals encounters are easily the best chance to find a new recruit. These are also not gated behind a generic randomizer event like the crash site one, so it is absolutely recommended to always check these out.

While events can time out, it does not matter to the chances as to when you explore those sites. If you send an explorer shortly before the event expires, the chances are the same as if you sent them right away.

Note about the chances: The code is not always clear to interpret in this regard, sometimes numerical values are given, sometimes not, and sometimes partially (the worst), leaving a lot of room for interpretation.

Hope this is helpful to anyone.