surreptitious
sur-rep-ti-tious
adjective
Done secretly, often because it would not be approved of.
"She cast a surreptitious glance at his phone."
Thesaurus
Synonyms: sneak, furtive, concealed, stealthy, sneaky, hugger-mugger, secret, clandestine, hush-hush, cloak-and-dagger, undercover, covert