Board Thread:Fun and Games/@comment-26998507-20150927171529/@comment-17754282-20151019203513

Rexy is attracted to flares because she has learned that they signal food. Malcolm's flare led Rexy to Gennaro while in Jurassic World flares are lit near the goat so that Rexy comes for it. Over time this has become a learned behaviour for Rexy, probably overcoming her inability to see things that don't move - she sees the flare and knows that food is there. All she has to do is wait for it to move.

It's like how Owen says that the Indominus has come to associate its crane with food. Hearing the crane start up is probably all she needs to know lunch is on the way, even if she can't see the food yet.