The only way they could do it "without the drivers knowing" would be to load a "turned down" engine map in to the cars. This would have to be done before qualifying. If the cars were "turned down" in qualifying, Hamilton would not have been ahead of Vettel on the grid. Hamilton's good but even he can't make up for a turned down engine on a circuit with two big straights and the fastest top speeds this side of Monza.
If they turned down the engines during the race, and that seems reasonable once they knew they were safely in the correct places for the title, they would have had to tell both drivers to make setting changes. The drivers would know that this was a lower power map.