• Blew the whistle
• Cautioned
• Flagged
• Gave a heads-up
• Gave a heads-up to
• Gave a warning
• Gave fair warning
• Given a tip
• Given advance notice
• Made aware
• Notified
• Notified of danger
• On guard
• On one's toes
• Put on guard
• Sent signals to
• Sent smoke signals, maybe
• Signaled
• Sounded the alarm
• Tipped off
• Vigilant
• Warned