Furtive

/ˈfərdiv/

adjective

secretive, attempting to avoid being noticed, usually due to guilt
So swift, silent and furtive were his movements like those of a trained bloodhound picking out a scent, that I could not but think what a terrible criminal he would have made had he turned his energy and sagacity against the law instead of exerting them in its defense.
– from The Sign of Four, by Arthur Conan Doyle (1859 – 1930)