It's the holiday update. Included with this update is some general fixes along with a new option to give the monster a festive Christmas hat and sleigh bells.

Added
- Christmas mode which can be enabled in settings. Gives the monster a Santa hat and replaces his chain noises with sleigh bells.

Fixed
- Monster sometimes violently vibrating.
- Decreased attenuation radius for monster smashing doors in. Now it won't sound like its right in your ear.
- Monster forgetting to flee if its triggered before it spawns.
- Monster turning to face the wrong direction for patrols.
- some environmental geometry.