off the grid
on the Q.T.
hidelings
covertly
under the table
secretly
hole-and-corner
hidingly
clandestinely
clandescent
concealedly
surreptitiously
underhand
steal
secretively
off-the-record
hugger-mugger
sub rosa
underground
cloakedly
on the hush
secretolytically
confidentially
under the rose
backstair
back street
dark
privily
in secret
underhandedly
semisecretly
on the QT
shadowing
tailing
a man's home is his castle
closehanded
espionage
snooping
sensitive
latently
stalkily
black-bag
undisclosedly
on the down-low
under the covers
spying
black bag operation
hideling
behind-the-scenes
sub-rosa