It’s an old trick of mine and easy to mess up. Basically you hold the flame near but not on the plastic, ideally so you warm it but not burn it. Once it has softened you can run a wet finger over the area and it regains some of its sheen. I do the same for dealing with flash marks left from the factory sloppily taking parts off a sprue. Doesn’t always work depending on the softness and translucency of the plastic so your mileage may vary.