backstair
backstairs
behind the scenes
black
clandestine
cloak-and-dagger
cloaked
collusive
conniving
disguised
furtive
hole-and-corner
hugger-mugger
hush-hush
masked
protected
secret
sub-rosa
subterranean
subterraneous
surreptitious
ulterior
under wraps
under-the-table
undercover
underground
undisclosed
unrevealed
abri
abstruse
air-raid shelter
arch
armored
asylum
back-door
beclouded
between the lines
blanket
blind
bolt-hole
bomb shelter
bombproof
boscage
bosket
brake
bunker
buried
burrow
by stealth
cache
camouflaged
canebrake
cased
cave
ceiled
ceja
chamisal
chaparral
cloak
close
clouded
coat
coated
concealed
concealment
coped
coppice
copse
copsewood
corner
couch
cover
coverage
covered
bird
flock
coverted
covertical
covertness
coverts
deliberate
diplomatic
direct
indirect
mutual
ongoing
overt
sly
subtle
tactical