In the scene near the end, where Travis is beating up Barris, Travis had recently grabbed the blade of the knife when Barris tried to stab him. His hand was shown to be very bloody. However, when the red light and the alarm went on and Travis raised his hands to his head, his hand was completely clean and unharmed.
After using the fire extinguishers on the prisoners everyone is covered in white powder as if a powder apparatus had been used. The actual extinguishers used are of the CO2 type which is easily identified by the big "funnel" at the end of the hose and would not leave everyone covered in white powder.