The opposite of "are on alert" could be "are relaxed" or "are unaware." Being on alert means being vigilant, attentive and watchful. On the other hand, being relaxed means being calm, at ease and not worried. When someone is unaware, they are oblivious to their surroundings and are not paying attention. In some situations, it is important to be on alert, such as during emergencies or dangerous circumstances. But in everyday life, it is also important to be able to relax and enjoy oneself. However, it is important to find a balance between being vigilant and being at ease.