Student or Learner
Why do you say:
"The police are combing the area" but "The army is combing the area"?
'police' (noun) is always regarded as plural. The singular would be policeman/policewoman/police constable.
'army' is normal in formation, with the plural being 'armies'. Hence, in your sentence, it would be the equivalent of saying:
One of the military services is combing the area.