Other mistake: Towards the film end, when George climbs up stairs with the gun to kill others, he walks into the passage and shuts the door. A mirror is there in the passage, but there's no reflection of George in it. (01:15:15)
Other mistake: The type of pan George uses to make eggs is far more modern a design than when the movie is supposed to take place in the late 70's.
Answer: He sees the balloon, and instantly assumes his step-daughter is inside the boat house, and hence is in danger of slipping and drowning (or perhaps already in such a situation). He rushes straight out to get there as quickly as possible. Checking her room, or telling his wife would take up time, not to mention he is probably running on adrenaline.