Yes, there are going to be epoxy fumes which one doesn't want to directly inhale whether its warmed in water or in a microwave or just using the product.

A short 15 second warming is hardly going to out gas much, over doing it can cause problems. Any type of warming method like warm to hot water as suggested will do the trick. Setting the material on dashboard of a car in the sun will have the same result. The idea is to scramble the epoxy molecules by increasing the Brownian motion with heat. To much heat could certainly collapse the plastic container so caution is advised, as I gave warning.