Minefields have always been the tactician’s weapon. These explosives do not require constant supervision, nor must they be manually engaged. They are planted with the explicit purpose of detonating only when something has crossed the threshold of their view. Constant radar pings ensure that the surrounding area is under supervision at all times.
A minefield may be placed in multiples, creating either a surprise attack, or carpeting an area with them to ensure that nothing can emerge unscathed. The detonation system is imperfect as is the technology. Therefore, the odds of a mine exploding are based on the number of them that are present. One mine has a one in a hundred chance. Fifty has a fifty in a hundred chance. In this specific case, twenty five of the mines could remain undetonated. For the sound tactician, proximity mines should be a resource, but not a necessity as they cannot entirely be relied upon.