My island has about 91 inhabitants at this point, most of whom are married. As such, it is almost a daily event to have a baby is born or move out. Because of the limited space, the majority become travelers. I've noticed something odd, however. I have a few friends who also play the game, and we Streetpass constantly. Excitedly, we'll all whip out our 3Dses like a bunch of school children and start asking "Ooh, who got Misty?" "Did anybody get Johnathon?", referring to our travelers. Sometimes, however, no such traveler will be sent. They'll just...vanish. For example, a friend who streetpassed me saw her traveler "Baritone" get onto a boat, presumably to my island. But when I check my system, he isn't there. I've streetpassed her, and I certainly have enough room on the island for a traveler - the campground is wide open. So where exactly has Baritone been taken? It's happened a few times to some of my Miis, and while I have travelers to spare, I'm not sure exactly what's happening.
Bonus question - when your islander sends you a letter, and the game shows them sitting on what looks like a deserted island ("I can see some islands in the distance...maybe I'll visit them next!"), where exactly are they? On a player's system still, stuck in the limbo I mentioned above...?