The phrase "are on scene" refers to the presence of someone or something at a particular location. The antonyms for this phrase could be "not present," "absent," or "not there." For example, if there is a fire at a building and the firefighters are on the scene, their antonyms would be that they are "not present" or "absent" if they have not yet arrived. If the police officers are on scene at a crime scene, their antonyms would be that they are "not there" if they are not present. In general, the antonyms for "are on scene" would indicate a lack of presence or absence at a particular location.