The adjective furtive means someone who is behaving as if they want to keep something secret or hidden.
Synonyms are sneaky, clandestine, secretive, or covert.
The word origins from Latin (16th century) furtivus meaning “stolen”, hence also “hidden, secret”, from furtum meaning “theft, robbery, a stolen thing”, from fur meaning “a thief, extortioner”, also a general term of abuse “rascal, rogue”.
With a furtive glance over her shoulder, she sneaked through the gates of the old house.
They exchanged furtive gestures during the exam.
I’m cautious of people with furtive behaviour.