done secretly; kept secret

secret, sly

(of glances, threats etc) hidden; disguised

clandestine, underhanded

done secretly so as not to attract attention

sneaky, secretive

(that is to be) kept from the knowledge or view of others; of which others have no knowledge

hidden, unrevealed

underground

hidden, underground

(of actions) done secretly

sneaky, secret

beyond what is first seen or said

secret; pertaining to a hidden goal

acting secretly

secret, spy

deceitful(ly)

deceitful