Surreptitious

ˌsʌr.əpˈtɪʃ.əs

adjective

kept secret, especially because it would not be approved of

The word 'surreptitious' comes from the Latin word 'surrepticius' meaning 'stolen' or 'sly'. It implies something done in a secretive or sneaky manner.

I even forbore to ask her why it was necessary to abstract what she wanted in such a surreptitious manner.

Agatha Christie

The Murder of Roger Ackroyd

It was early and Alia had not yet arrived, but people already were thronging into the chamber, casting surreptitious glances at the child and adult as they passed.

Frank Herbert

Children of Dune