Simple:
* The spy surreptitiously slipped a listening device into the diplomat's briefcase.
* She surreptitiously checked her phone during the meeting, hoping no one would notice.
Adding context:
* The thief surreptitiously pocketed the diamond necklace while the shop owner was distracted.
* The child surreptitiously grabbed a cookie from the cookie jar, hoping to avoid getting caught.
More complex:
* The journalist surreptitiously gathered information from confidential sources, knowing that his actions could put him in danger.
* The dog surreptitiously snatched a piece of pizza from the counter, hoping to avoid the wrath of its owner.
Figurative:
* The government surreptitiously manipulated public opinion by spreading propaganda.
* The artist surreptitiously infused his paintings with hidden messages, only visible to those who knew what to look for.
Remember that "surreptitiously" implies secrecy and often carries a negative connotation, suggesting that the action being described is done in a sneaky or clandestine manner.