WordNet sense 1 (not existing):

missing, nonexistent, lacking, wanting

WordNet sense 2 (completely lacking):

devoid

WordNet sense 3 (not present; having left):

away, gone

WordNet sense 4 (go away or leave):

remove

WordNet sense 5 (lost in thought; showing preoccupation):

absentminded, abstracted, scatty

WordNet sense 6 (showing a lack of attention or care):

inattentive