disguised — to change the appearance or guise of so as to conceal identity or mislead, as by means of deceptive garb: The king was disguised as a peasant.
clandestine — Something that is clandestine is hidden or kept secret, often because it is illegal.
mystic — involving or characterized by esoteric, otherworldly, or symbolic practices or content, as certain religious ceremonies and art; spiritually significant; ethereal.
secret — done, made, or conducted without the knowledge of others: secret negotiations.
stealthy — done, characterized, or acting by stealth; furtive: stealthy footsteps.
surreptitious — obtained, done, made, etc., by stealth; secret or unauthorized; clandestine: a surreptitious glance.