Are you referring to his entrance into Dol-Guldur? He went to "spy" and see if the Necromancer was in fact the Dark Lord Sauron come back to wage war once again upon Middle-earth. I cannot think of any other "spy missions," unless you mean tracking down Gollum.
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...where the instrument of intelligence is added to brute power and evil will, mankind is powerless in its own defence.
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