Evaporation of bad quality gasoline?

I am a gas station owner. I got my tank refilled a few days ago. The day after the refil there was about 40 liters missing from the tank. The tank was not used that day and the leak test came ok. Also I am quite positive that nobody stole the gas from the tank.

The next logical conclusion is that they gave me a batch of bad quality gas that evaporated quickly. Is that possible?

The tank has a capacity of 15000 liters and when that happened it was almost full at about 14000 liters

40 litres out of 14,000 is about one third of one percent - doesn’t seem like too huge an amount to lose by evaporation - but it doesn’t seem like too huge to be within the margin of measuring error either. I don’t know what’s normal though…

Have you taken thermal expansion into consideration? If the temperature of the gas dropped a few degrees from the first to the second meassurement that could account for your discrepancy.