clandestine

clandestine \klan-DES-tin\, adjective:

Characterized by, done in, or executed with secrecy or concealment, esp. for purposes of subversion or deception.

One of the many shiny art panels at the back of the room is actually a clandestine two-way mirror (look carefully, the color is slightly different). Back in the day (perhaps now) it allowed managers to survey service and presentation, which are still impeccable.
-- Ike DeLorenzo, Five classics revisited, Boston Globe, 27-May-09

I was commanded by Paramount's publicists -- the Legion of Women With Clipboards -- to come alone to an advance, clandestine screening of "Star Trek" a couple of weeks ago.
-- Hank Stuever, The Trouble With Quibbles, Washington Post, 11-May-09

They can also stealthily enlist a computer into so-called botnets - computers that have been clandestinely networked to perform tasks without the knowledge of their owners and operators.
-- Scott Duke Harris, Internet security problems have an upside for Silicon Valley, Mercury News

Clandestine is from Latin clandestīnus, probably a blend of clam-de, secretly and intestīnus, internal.