During trauma resuscitation, two patients with blunt abdominal trauma are being evaluated. Patient A has a positive FAST (free fluid in Morrison's pouch and pelvis) but is haemodynamically stable after 2 litres of IV crystalloid. Patient B has a negative FAST but is haemodynamically unstable (SBP 85 mmHg, HR 128 bpm) despite 3 litres of crystalloid and ongoing transfusion. Which finding best discriminates between these two presentations and determines the need for immediate laparotomy?