Signs

Graham Hess (Mel Gibson), a former priest, lives with his asthmatic son, Morgan (Rory Culkin), his daughter Bo (Abigail Breslin) who leaves glasses of water around the house for various reasons, and his younger brother Merrill (Joaquin Phoenix), a failed minor league baseball player, on an isolated farm in Doylestown, Pennsylvania. Hess has lost his faith and gave up his priesthood after his wife, Colleen, died in a traffic accident caused by a town local, Ray Reddy (Shyamalan).
One morning, Graham discovers a crop circle in his field. While investigating the crop circle with the local law enforcement, Morgan is forced to kill one of the family dogs when it turns violent against him and Bo. Graham then learns of animals becoming violent across town. They hear reports of crop circles appearing around the globe, and there are reports of strange lights over Mexico. Later, Graham and Merrill find the farm is being watched by a tall dark figure which disappears into the fields when they give chase. The next day, they hear strange noises over a baby monitor, but cannot find their source. Graham approaches the crop circle but on seeing a green leg sticking out of the cornrows, flees back to the house.
After opening up to Merrill about the death of his wife, noting that her last words were "tell Merrill to swing away", Graham goes to visit Ray, the first time since the incident. When Graham is gone, Merrill and the children watch a news report about footage taken of an alien creature in Brazil, showing that the aliens can blend into their surroundings. Merrill joins the children in wearing tin foil hats, believing the aliens can read their minds. At Ray's, Graham finds Ray is bleeding from an injury but otherwise apologetic for the accident. After Graham accepts his apology, Ray departs for a nearby lake, saying "they" do not like water. Graham finds Ray has trapped one of the aliens in his pantry, and tries to see what the alien looks like using a kitchen knife under the door; the alien grabs at him, and Graham reacts by cutting off its fingers before fleeing. On his return, and hearing news that more lights have been seen across the globe, the Hess family decides to stay at their house instead of leaving for the lake, barricading the doors and windows.
After an emotionally charged dinner, they hear the same noises on the baby monitor before their television signal drops out, and place the final barricades and defenses before they gather together. As the aliens attempt to gain entry into the house, the family discovers they have left the attic door unguarded and quickly retreat to the basement. The aliens assault the basement door and in the subsequent chaos the light goes out as Graham and Merrill prevent their entry. However, Morgan suffers an asthma attack when the aliens attempt to enter by way of an old coal chute. By morning, radio reports claim that the aliens have left Earth as suddenly as they arrived. Graham leaves the basement to locate Morgan's asthma inhaler, the others following him. But they discover an alien, the same that Graham had attacked in Ray's home, is still there, and takes Morgan hostage. Colleen's last words come back to Graham, and he instructs Merrill to "swing away" with a baseball bat. Merrill attacks the alien but not before it attempts to poison Morgan by spraying him with a toxin from its body. Graham recovers his son when the alien drops him and takes Morgan outside with his medication. Merrill continues to attack the alien, causing it to fall onto the ground, knocking over one of the glasses of water Bo had left and splashing it with water. The water reacts like acid on its skin, making it scream in agony. Merrill strikes the alien against furniture where it is doused with water full on its face, and dies. Outside, Graham administers Morgan's medicine and waits for his son to wake up, hoping that due to his lungs being closed from his previous asthma attack he was spared. Then, Morgan wakes up as the family rejoices.
Some time later, the Hess family has recovered from the incident and they appear to be doing much better than before. In the final scene, Graham is shown returning to his priestly duties, apparently having regained his faith.

Question: what did the family discover as the aliens entered the house?
Answer: That they had left the attic door unguarded.