Burglars broke into a building next to Sarratt Post Office in the early hours of Sunday morning.
Hertfordshire Constabulary were alerted to the crime after an alarm was triggered at 12.29am. A concerned neighbour also contacted the police 10 minutes later.
The criminals forced entry to an area at the side of the main post office, in The Green.
Nothing has been reported stolen.
Anyone with information can contact the police on 101 quoting reference C2/14/293.