Hoopy is known to appear in a few places within Portal 2. Its earliest known appearance is in GLaDOS' ruined chamber, where it lays slightly to the left of what could be described as GLaDOS' head; its second known appearance is underneath a floor panel in Test Chamber 02. It then makes its third known appearance behind a desk in Rattmann's den in Test Chamber 06; this is the same den as referred to by the Final Transmission achievement.
Hoopy's fourth appearance is in one of the randomly-chosen exit corridors in chambers created with the Perpetual Testing Initiative. It is seen by noclipping into the ceilings of the exit corridor, although it doesn't appear in every one.