Wiktionary
flag up
vb. (context transitive English) To mark for attention; to flag.
Usage examples of "flag up".
He turned to his stunned commander and bade the man to run the white flag up at once.
If I wave the flag up and down on my right side, the enemy force is off to the right, and if I wave it to the left, the danger's there.
He, in turn, ran a flag up the halyard running to the height of the stem turret.
If I wave the flag up and down on my right side, the enemy force is off to the right, and if I wave it to the left, the dangers there.