The mass shooting Wednesday at a Walmart in Virginia is only the latest example of a workplace shooting perpetrated by an employee. Many companies have active shooting training. But experts say there is much less focus on how to prevent workplace violence. Workers too often don’t know how to recognize warning signs and co-workers. More crucially, they often don’t know how to report suspicious behavior or feel empowered to do so, according to workplace safety and human resources experts. One expert said too often attention is focused on the “red flags” and workers should instead be looking for the “yellow flags” — subtle changes in behavior, like increased anger or not showing up for work.