noun (n)

• keeping a secret or furtive watch(noun.act)

  source: wordnet30
• the act of keeping a secret watch for intelligence purposes(noun.act)

  Synonym:
  undercover work
  

  source: wordnet30
• the act of detecting something; catching sight of something(noun.act)

  Synonym:
  catching, detection, espial, spotting
  

  source: wordnet30