I haven't seen anything in the rules that requires a statement of intent. But in the combat example, the game master asked each adventurer "what they intend to do in this first round of combat." But she didn't hold them to it, because she allowed Thrace to change his actions when Lilina was seriously wounded by the trollkin.
I would generally allow a player to react to the things going on around them like this. I might require some sort of "situational awareness" roll if there was a question of whether the character would know what happened to react to it. Since I have only run a couple of trial sessions with friends, it hasn't been an issue yet.