I used the 'blank cheque and honesty box' system once at Brize. It was Christmas holiday time and the cheque was never cashed - so I guess they gave me a little present!
A few people might have been a little inaccurate with the value of their personal purchases, but I don't recall hearing of anyone being stupid enough to try smuggling. At least, not on 101 Sqn. But once I asked an Oxford CAA examiner if he could do my civil FI revalidation test after he'd finished for the day - to which he agreed. He turned up in his dark blue NATO pullover with 4 gold bars and came over to the Flying Club. Which at the time was right next to the Air Cargo hangar. This caused mayhem amongst the Movers, so I asked their genial Flt Sgt what was the problem. "They think that the senior customs officer from Heathrow has just turned up to direct his staff to do a rummage of the hangar". "Shall I ask him to take off his captain's rank slides?", I asked. "Please don't do that sir, my lot are panicking and giving the hangar a good tidying up - I've been waiting for them to do that for months!".
But some fork lifts were seen carrying clinking cardboard boxes off to various other storage areas.....
Some people are mind-blowingly stupid though. I was sitting on the patio at Akrotiri OM once and couldn't avoid hearing a loud mouthed Hercules navigator bragging about the profits he'd made selling 2 x 1.75 litre jugs of Vodka every time he came back from some exercise trips. Bought in a BX for low price and sold to the locals back in the UK....
Sorry - as was often said "It ain't worth my pension!".
Another criminal trick I heard about was something which went on at Dulles. After crew-in, some mover would open the cargo door and close it again "It wasn't properly shut". When the aircraft landed at Brize, the conspirator would be first to open the cargo door to retrieve whatever his crony had hidden at Dulles....
Always be suspicious if a door light comes on briefly and goes out again after the steps have been taken away, was my advice... Phonepatch ahead and get the RAFP to meet the jet, if necessary.