bugging
chaff
chasing
cloak-and-dagger work
counterespionage
counterintelligence
dogging
electronic surveillance
espial
espionage
following
heeling
hounding
intelligence
intelligence work
military intelligence
observation
pursual
pursuance
pursuit
secret police
secret service
sequel
sequence
series
shadowing
spying
stakeout
surveillance
trailing
wiretap
wiretapping
tail
tails
chase
ailing
assailing
bailing
baling
curtailing
derailing
detailing
emailing
entailing
failing
flailing
grayling
hailing
impaling
inhaling
jailing
mailing
nailing
prevailing
railing
regaling
sailing
scaling
surveilling
unavailing
unfailing
unveiling
veiling
wailing
whaling