A wallet, PlayStation game and a laptop computer were stolen from a house in Main Street, East Farndon.
It happened after the burglar pushed the key out of the lock and reached through the cat flap to pick it up off the floor.
The laptop is a Dell Inspiron make and model with the letter ‘N’ missing.
Police have urged people never to leave keys in locks as they can be accessed through an opening such as a letterbox or, as in this case, a cat flap.
They appealed for witnesses to the break-in, whihc happened between midnight and 6.40am on Thursday, October 10, to contact them.
Anyone with information should call Northamptonshire Police on 101.
Alternatively, information can be given anonymously to Crimestoppers on 0800 555 111.