Same word as backstairs:

backdoor, backstairs, camouflaged, classified, close, confidential, covert, cryptic, discreet, disguised, dissembled, dissimulated, furtive, hush-hush, in ambush, incognito, inside, restricted, secretive, sly, sneak, sneaky, stealthy, sub-rosa, surreptitious, top secret, unacknowledged, under false pretense, under-the-table, underhanded