Hedda didn't fully understand the nature of the threat, but she wasn't curious enough to voluntarily advance into a danger zone either. Her training as a spy had included lessons about observing from safe positions.
She hid among the non-abyssal soldiers, trying to go unnoticed as she had been doing throughout the battle.
Master Vex's collar continued functioning effectively, keeping her invisible to most casual observers.
But she ended up seeing what was happening when the infection finally reached the "wall of abyssal soldiers" that divided the bridge entrance, the point where they had precisely separated the last common soldiers from those with corruption.
A soldier completely covered in golden spores who was trying to flee desperately from the bridge was hugged by several abyssal tamers who screamed upon making contact. Strange smoke emerged from the contact points, the person fainted, but there were no major complications.