Happened to me!!!
I stopped at my friends restaurant to check my barrels and have a sandwich and a beer. The drums where full almost to the rim. I told him I would be back in the morning to pump them out.
Arrived there early the next day, set up my pump and barrels that I pump into, went over opened the lids and found nothing!!! 110 gallons gone, they pumped them dry!! I was pi$$ed to say the least.