Between a goat's normal digestion and the length of time it was was left unsupervised in Greg's bedroom, the floor & bed would likely have been covered in feces, yet there was none in any of the scenes.
People in the linen closet can hear Greg speaking in the attic through the AC duct, but Greg in the attic can't hear people in the linen closet when they speak.
When Greg first takes the goat up to his room, the goat is heard bleating but it's mouth doesn't move. This happens more than once.