abstruse
ambiguous
arcane
camouflaged
charge
chargeless
classified
cloakanddagger
close
closet
comp
complimentary
concept
confidential
costless
covered
covert
cryptic
dark
deep
disguised
esoteric
for love
for nothing
free
free of cost
free ride
freebie
furtive
gratis
gratuitous
grunge
grungy
handout
hidden
hushhush
ink
less
miscellaneous
mysterious
mystic
mystical
obscure
occult
on the cuff
on the house
outoftheway
paper
private
quiet
rectangular
red
rubber
secluded
secret
sign
stamp
symbol
undercover
underground
undisclosed
unknown
unpaid
unprovoked
unrecompensed
unseen