Why is __ember_meta__ getting added to my vanilla javascript ember-data property?

I have a model that has an attribute I define as follows:

aux: Ember.DS.attr()

So there's no transformation going on. And when I first load the model, when I do a model.get('aux'), the result is a vanilla javascript object.

But somewhere along the line my application is doing something that adds a bunch of extra properties to the object like __ember_meta__. This is interfering with my attempt to recursively clone the object, because it enters an infinite loop.

I'm not too concerned about my infinite loop problem, but rather I'd like to understand under the hood what I'm doing that's causing Ember to transform my vanilla javascript object.

It's not an Ember.Object instance, at least according to Ember.Object.detectInstance(aux).