Over the last two days there have been several Burglaries where cars have been stolen overnight as a result of the Burglars stealing the car keys from within the house. 

The point of entry on every singe incident has been by inserting an implement through the letter box and opening the front door from within.
This is basic Crime Prevention that has been mentioned before on many occasions.  Please get into the habit of securing your front doors at night, if you can open it by just pulling the catch down yourself then so can someone from outside if they are able to access the catch.
Solutions to help are, double locking the door, using the door chain (if Fitted), installing a letterbox deflector, sealing the letterbox and having one on the wall. I am sure there are many others but this needs to be a long term plan, don't just do it for the next week because it is happening now as these trends are repeated throughout the year.  Also, do not leave keys downstairs or close to your front door, take them to bed with you and keep them in a bedsite drawer or similar