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The retailers who kept the tickets
A sting found shop staff claiming customers' winners at rates nobody expected.
Several jurisdictions have tested this by sending investigators into shops with winning tickets and asking staff to check them. The results have been bad enough to make the news more than once.
Statistical work has also found retailers claiming prizes at rates far above chance — in one analysis, a small number of clerks had claimed dozens of winning tickets each, which is not something that happens to people who merely play a lot.
The countermeasure is now standard: customer-facing screens on terminals, and prompts telling you to sign the ticket before handing it over. Both exist because of this.
The statistical approach that exposed it is elegant: look at how many winning tickets each individual claimant has cashed over the years. A handful of people with dozens of wins each is not a run of luck, and it does not require catching anyone in the act to spot.