Surreptitious: kept secret, especially because it would not be approved of. From Latin surreptitius: secretly and seize.
Sneaky: furtive, sly, reluctant.
Wily: skilled at gaining an advantage, especially deceitfully.
Clever: quick to understand, learn, and devise or apply ideas; intelligence. Derived from Middle English perhaps of Dutch or Low German origin. In the late 16th century, the term came to mean manually skillful. The sense of possessing mental agility dates to the early 1700s.