I have seen this in the marine environment when we were taking on bunker fuel in Korea from barges. The Chief Engineer noted over two tonnes discrepancy between what was on the paperwork and what his engineers calculated had been pumped onboard. He knew his engineers wouldn't make a mistake that big, so he got suspicious and came up on deck at the base of the accommodation-block where our bunker-tank manifold connection was. With him was the Second Engineer and the Agent from the shipping company who represented us in Korea.
The Chief refused to sign for the fuel. The Korean barge-master then said "maybe some stick to sides" and a pump started somewhere on the barge. 0.8 more tonnes of fuel-oil came aboard.
The Chief now knew how the scam worked. There was a hidden tank on the barge and a small quantity of fuel was siphoned into it as the main body of fuel was pumped via the manifold into our bunker-tanks. He figured if there was 0.8 tonnes more that had just mysteriously "come down from sides" then the rest was still onboard the barge. So he still refused to sign the paperwork.
The Koreans began to get very unsettled. Twenty minutes of haggling between the barge-master, the Agent and the Chief Engineer and still we didn't have all the fuel. The Agent was getting some dirty looks from the barge-master, and the Chief was standing his ground, a smile grdadually developing as he saw the evident discomfort of the barge-master. Then the barge-master went below on his barge, came back a minute or so later and pressed a small piece of paper into the Chief's hand. The Chief turned away and looked at it, then signalled the Second Engineer over to have a look at it. Apparently it read (in English, surprisingly) "How about we give you little present?". A bribe was about to be offered.
Now the Chief's suspicions were concerned. It was a scam and the barge-master was probably going to be in trouble with Korean Customs (who would have to be in on the scam anyway) if the barge came back without the two tonnes of illicit fuel-oil.
The Chief asked the Agent to tell the barge-master in Korean "No Deal"...I want my fuel and I want it all, and I want it all now." We weren't sure if the Agent was also in on the scam, but he did as he was asked and the barge-master scuttled away, muttering in Korean. The asthmatic pump ground into action on the barge again, and a further 1.1 tonnes of fuel-oil arrived onboard.
By now the Korean barge-master knew he had been out-stared by the Chief and all he had for his trouble was maybe a couple of hundred pounds of fuel-oil instead of the couple of tonnes he was hoping to spirit away. The Agent stayed aloof, the Chief signed the paperwork and the barge departed.
I watched a little of the drama but was called away. The full story was related to me that evening in the smoke-room by the Chief Engineer, who still had the little piece of paper. We all had a good laugh about it, but there is a good chance the barge-master got his backside kicked royally by the Customs guys with whom he was probably hoping to split the profits from selling the purloined fuel. I guess those Korean Customs guys could kick pretty hard, especially one of their own.
So, such scams are alive and well. All it needs is a way to siphon off a small volume as the main load of fuel is transferred, and a hidden tank in which to store it so it can be taken for subsequent "re-sale". I don't think that would be all that hard to do on a fuel-truck.
As Clarkson says "How hard can it be?"