Definition: Acting stealthily, sly and secretive; under cover, out of view.
Usage: The verb, surrept, means "to steal or filch stealthily" and the noun is surreption "theft by stealth" or "secretive misrepresentation." Both are highlighted now by my spellchecker, indicating they are used far less often than today's adjective. Remember that today's adjective is written "surreptitious" with two "t's" and without any "c."
Suggested Usage: "Connelly thinks someone surreptitiously replaced everything in his office with exact duplicates." (Don't laugh at Connelly; the same thing happened to me!) The real challenge to all of us, however, is to revive the relatives of today's word: "Madeleine rose to the top of the company through surreption and intrigue." If the police in your neighborhood tend to ignore you, try this to capture their attention: "Someone surrepted my car under cover of night."
Etymology: Latin surrepticius from surripere "to take away secretly" based on sub- "under" (used in many words to mean 'under cover, secretly') + rapere "to seize." We Indo-Europeans have done well for people whose ancestors apparently struggled to distinguish up from down. The Latin prefix sub- "down, under" comes from the same PIE root that gives us English "up." The original root was *upo but PIE words often appeared with and without a mysterious initial "s." Even in Latin, super- "over" comes from the same root with a suffix –r while in Greek hypo- "under" (hypodermic "under the skin") and "hyper- "over" (hypersensitive) share the same origin as Latin sub- and super-. (See our FAQ sheet for more PIE.)
–Dr. Language, YourDictionary.com
People with teen age children will recognize this behavior but didn't know the word. AKA sneaking around.....