False Objects. If the swarm is motionless at the start of combat, it has advantage on its initiative roll. Moreover, if a creature hasn't observed the swarm move or act, that creature must succeed on a DC 15 Wisdom (Perception) check to discern that the swarm is animate.

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a 1-foot-tall, 8-inch-wide, 2-inch-thick object. The swarm can't regain hit points or gain temporary hit points.