The adjective surreptitious means something that is done secretly, without anyone seeing or knowing.
Synonyms are clandestine, covert, furtive or unauthorized.
The word origins from Latin (middle 15th century) surrepticius meaning “stolen, furtive, clandestine”, from surreptus, past participle of surripere meaning “seize secretly, take away, steal, plagiarize”. It comes from sub meaning “from under” plus rapere meaning “to snatch”.
Their surreptitious glances at the audience made them feel more nervous.
Pia helped her classmate to complete their assignments in a surreptitious manner.
He made a surreptitious recording to catch them in the act.