adumbration
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
foreshadowing