A pre-hospital team is managing a 60-year-old male who presents with acute onset of severe upper abdominal pain radiating to the back, nausea, and vomiting. He is an alcoholic. He has epigastric tenderness with guarding. His serum amylase (on point-of-care testing) is 1200 U/L (normal <100). He develops Grey Turner's sign (flank bruising) during the assessment. What does Grey Turner's sign specifically indicate?