Carefully observant or attentive; on the lookout for possible danger. Synonyms:open-eyed, vigilant, wakeful."The vigilant eye of the town watch" , "There was a watchful dignity in the room" , "A watchful parent with a toddler in tow"
... had employed it on occasion to hoodwink Mrs. Grundy for Mrs. Hilliard's sake, scrupulously meeting and leaving the lady outside the corporation limits, a ruse which deceived nobody save the deceivers. Nor was it effective now. Ruth passed Mrs. Bowers's argus-eyed bay window, as did Shelby, and Mrs. Grundy had her ... — The Henchman • Mark Lee Luther