Surreptitiously is the adverb: it tells you HOW someone did something — secretly and quickly, hoping nobody would see. You slide it next to the verb: "He surreptitiously checked the time." "She glanced surreptitiously at the door." Because it describes an action, it is often used with verbs of looking and small movements — glance, check, slip, take, pass, remove. The person is doing something a little bit not-allowed and does not want to be caught, though usually it is minor and even a bit funny. This is a formal, written word. In real speech, most people say "secretly", "quietly", "on the sly", or "without anyone noticing". You will meet surreptitiously most often in novels and news writing, where the writer wants one precise word instead of a phrase.