Mimic city; every building is a mimic, but they're smart enough to eat the local population when they're alone or in small groups. Every time people keep going missing, and the players will assume it must be some random wandering monster; they'd never think it's literally the buildings and each room can start eating really fast. There'd be no pattern either, every building is alive and people go missing literally everywhere. Perhaps a clue can be a house that has no more people living in it and it starts to starve to death and starts freaking out, even endangering the other mimics' secret.