Set the scene
Keep the sea in the background rather than describing every wave. The postcard is a small invitation to compare perspectives without forcing a mystery to become dramatic.
Choose what happens next
Choose one discrepancy: an extra doorway, a missing chair, or a distant island. Leave the explanation open.
An opening to borrow
“A postcard arrived with a picture of this terrace, but the view is different. Which detail would you investigate?”
Make the scene yours
Choose a pace that suits you. Add one observation of your own rather than specifying every reply. The characters are fictional adults, and the setting can stay entirely invented. Keep personal addresses, private histories and identifying details out of the exchange.
A natural ending
The characters can put away the object, change their plan, or decide to stop for the day. You do not need an elaborate conclusion. A short scene is complete when you no longer want to add another turn.