I have had many experience with DFT inspectors as a freight forwarder, they arrive unnanounced and will try to gain access without being seen, thats their job.
I am aware of an inspector arriving at an unamed transit shed in the North and ringing the manager of the shed and announcing himself and enquiring how secure his shed was from intruders. When he answered very he was invited to go into his warehouse and chat to said inspector who was sat ontop of a wooden case in the middle of the warehouse.