Simple:
* The thief made a surreptitious exit, slipping out the back door unnoticed.
* She cast a surreptitious glance at her phone, trying to hide it from her boss.
Adding Detail:
* He surreptitiously slipped the love letter into her pocket, hoping no one would see.
* With surreptitious movements, the spy retrieved the hidden documents from the safe.
Figurative Language:
* The wind carried a surreptitious whisper through the trees, rustling the leaves with an unsettling sound.
* The government's surreptitious actions were exposed by the investigative journalist.
Formal Tone:
* The detective used surreptitious methods to gather evidence without raising suspicion.
* The historian uncovered surreptitious communications between the two countries during the war.
Remember: "Surreptitious" implies secrecy, stealth, and often something hidden or dishonest.