FURTIVE

marked by stealth

covert

surreptitious

marked by quiet and caution and secrecy
"A furtive manner"

secret and sly or sordid
SYN - backstair
"furtive behavior"

"He had never appreciated the furtive way that the others went about things."

"he had always been so open and honest, which is why her furtive behavior was cruel, unfair and undeserved."

"Sure it wasn't easy to be so furtive about the party, but she knew she had to if it was going to be a surprise."