Pressure drop, what causes it?

Ok, lets say a student meausres the m of an empty metal tank as 38.2 kg. She adds 8.7 kg of tetra fluoroethylene gas to the tank and notices the pressure gauge reads 6.5 x 10^6 Pa. The next day, she notices the pressure reads almost 0 Pa but the tank's m is still 46.9 kg. What might have caused this drop in pressure?