They do happen in Newcrest—one of my sims was nearly abducted before I intervened with the resetsim cheat and put a stop to it. When I retraced the chain of events to the sim in question meeting and befriending an alien, said alien got marked for death (and left a really cool-looking headstone in the dump...er, graveyard.) Sure, the game just spawned another alien, but I found them in Manage Households and made their disguise look like their alien form for easy identification. My simworld's canon is that Earth is full of xenophobes, and aliens are hated and avoided, forbidden from holding jobs or marrying humans. No real "social commentary" implications, aliens were just an unwelcome addition to a pack I bought for the active careers.