Synonyms and related words :
ambuscade, ambush, ambushment, blackening, blind, blocking the light, booby trap, bugging, chase, chasing, cloak-and-dagger work, clouding, counterespionage, counterintelligence, darkening, dimming, dogging, eclipsing, electronic surveillance, espial, espionage, extinguishment, follow, follow-up, following, heeling, hounding, hue and cry, hunting, intelligence, intelligence work, lurking hole, military intelligence, obfuscation, obnubilation, obscuration, obscurement, observation, obumbration, occulting, overcast, overclouding, overshading, overshadowing, overshadowment, prosecution, pursual, pursuance, pursuing, pursuit, quest, searching, secret police, secret service, seeking, sequel, sequence, series, shading, spying, stakeout, stalking, stalking-horse, surveillance, tailing, tracking, tracking down, trailing, trap, wiretap, wiretapping