Justin had not received a detailed map of the minefield. In fact, the British didn't have one either.
The British simply laid mines according to a general area and considered it done. They knew where the minefields were, yet even they dared not enter.
Logically, during mine laying, one should record the positions of the mines to facilitate their removal after the war. However, logic is just that, logic. If that were actually practiced, the world wouldn't be left with so many minefields that were decades old and still deadly.
The British would fight a battle and then leave, unconcerned with the fate of the locals, so long as their own forces did not step on the mines.
For Gao Yang, knowing the safe areas was all that mattered. The task of mine removal after the war wasn't his responsibility.
There were no mines on the beaches of Gazaley City, nor outside the city to the northeast. The British were too pressed for time to lay mines everywhere.