The phrase "to flag up" means to draw attention to something, often by raising a warning sign or signal. This expression is commonly used in the context of identifying potential problems or dangers and taking steps to address them before they become more serious issues. For example, if an employee spots a safety hazard on the job site, they may "flag it up" by placing a caution sign near the area so that others can be aware of the danger and take appropriate precautions. In this sense, "to flag up" means to bring something to attention in order to prevent harm or damage.