to steal a glance (at); to look furtively (at); to take a peek (at)
surreptitious glance; looking furtively
taking (a picture or video) secretly; taking a sneak shot; making a secret (video) recording; filming without someone's knowledge
reading (someone else's letter, diary, etc.) surreptitiously; reading secretly; reading over someone's shoulder
chuckle; snigger; laughing secretly
making a secret audio recording