> On April 15, one trader made $21,000 (£15,600) betting that the maximum temperature would not be 18 degrees, data from Polymarket show. A temperature of 18C was seen as a 99.6pc probability before the temperature spiked later in the day.

I believe that these are the specific Polymarket bets in question:

April 6 (https://polymarket.com/event/highest-temperature-in-paris-on...): this appears to have been something of a "test run", since the odds weren't particularly lopsided until it suddenly spiked in the evening local time.

April 15 (https://polymarket.com/event/highest-temperature-in-paris-on...): the odds were quite lopsided in favor of 18°C, until suddenly reversing in the evening. This is presumably where the fraudsters made all their money.