surreptitious

Obtained, done, or made by clandestine or stealthy means. synonym: secret.

Done by stealth, or without legitimate authority; made or produced fraudulently; characterized by concealment or underhand dealing; clandestine.

Acting in a crafty or stealthy way; guilty of appropriating secretly.

Done or made by stealth, or without proper authority; made or introduced fraudulently; clandestine; stealthy.

stealthy, furtive, well hidden, covert (especially movements)

conducted with or marked by hidden aims or methods

marked by quiet and caution and secrecy; taking pains to avoid being observed