As the rulebook (page 9 – “Laying Mines”) states out: “Once laid a minefield remains active for the entire game”. More than once you will also find the phrase: “triggers the mine”. Does “minefield” and “mine” mean the same thing in this paragraph? I think so. But … What if there are actual two separate things?
The minefield piece defines the limit of the minefield and the design with five symbols indicates that it covers five squares on the Deep board and not that it consists of just five mines. The number of mines in the field is indeterminate and so, as the rules state, the minefield remains active throughout the game.