"Flag up" is an idiomatic expression that means to bring something to someone's attention or identify a problem or issue that requires attention. Some synonyms for "flag up" include "point out," "highlight," "bring to light," "raise awareness," and "call attention to." These phrases are useful in emphasizing the importance of an issue or problem that needs immediate attention. Other synonyms that can be used in place of "flag up" include "alert," "warn," "notify," "draw attention to," and "indicate." Whatever the synonym might be, these expressions serve the same purpose of drawing attention to important issues and problems that need to be addressed.