Anomaly drops an Anomaly Loot Bag.
Unlike the Grim Miniboss or Normal Bosses, Anomaly's spawns are infrequent (around 17-minute intervals) and spawns in 7 different locations throughout the backdoor (Swamplands, Winter Tundra (Bridge), Mushroom Forest, Twisting River, Farm, Spaceship, and Barn). Along with that, there's a notification in the chat when Anomaly spawns.
Anomaly has 2 "faces". When idle, it's face is a timer, but upon being vacuumed, becomes a smiley face.
The order of where the Anomaly spawns is as follows: Swamplands, Winter Tundra (Bridge), Barn, Farm, Twisting River, Spaceship, Mushroom Forest then back to Swamplands.