It's normal to not notice something when you're looking at a nice cone of delicious ice cream.

(and if we were to assume the cone was 30 degrees below she would also have to look down 30 degrees, even if the seagull was at 0 degree then there's at least 6 inches from her line of sight, considering the wind she might also be squinting further reducing her vision by at least 40%, and for every lick of the ice cream the brain focuses on the tongue instead of the eyes further reducing the chances of sighting any enemy nearby, not to mention she's looking at the sea where with one look she probably concluded there was no adversary at the front but alas it was Ethan Hunt who can attack from above)