WordNet (r) 3.0 (2006):

counterespionage
    n 1: spying on the spies

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "counterespionage":
   bugging, cloak-and-dagger work, counterintelligence,
   electronic surveillance, espial, espionage, following,
   intelligence, intelligence work, military intelligence,
   observation, secret police, secret service, shadowing, spying,
   stakeout, surveillance, tailing, trailing, wiretap, wiretapping