Ambush

Ambush is the entity that comes back. It charges through the rooms, reaches an unopened door, passes through it and then turns around to run the same rooms again, up to three times before it despawns. Each pass deals 100 damage to any player it can see, enough to kill with default health.

When it appears

Its chance grows as you progress. In The Hotel it can show up from Door 3 to Door 86, with the highest likelihood between doors 82 and 86; in The Mines it is less likely and runs from Door 103 to Door 197. Most full runs meet it about once, but that is not guaranteed. A high-pitched distorted scream announces it and the lights in the next room break. Entering The Dining Hall or The Bridges despawns it.

Dealing with the rebounds

Hide only while it comes toward you, leave once it moves away and be ready to hide again. Like Rush it uses raycasting, with a slightly longer reach, so standing out of its line of sight also works if you keep your distance. In The Mines switch hiding spots between passes, since Hide attacks you in the last one you used. A Crucifix banishes Ambush, so save it for when you struggle to hide.