Vacuum activities are innate, fixed action patterns of animal behavior that are performed in the absence of the external stimuli (releaser) that normally elicit them. This type of abnormal behavior shows that a key stimulus is not always needed to produce an activity. Vacuum activities can be difficult to identify because it is necessary to determine whether any stimulus triggered the behavior.
For example, squirrels that have lived in metal cages without bedding all their lives do all the actions that a wild squirrel does when burying a nut. It scratches at the metal floor as if digging a hole, it acts as if it were taking a nut to the place where it scratched though there is no nut, then it pats the metal floor as if covering an imaginary buried nut.
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The Daily Omnivore 