I just re-watched Signs. This time, I was looking at it from the perspective of "assume these may not be aliens". I thought that was fair, as the first time I saw it I went in assuming they were aliens.
I noted the following:
1) The little girl, and the mother, were having precognitive dreams about these events.
2) The boy's very first reaction, in the first 5 minutes of the film, to seeing the crop circles is "I think God did it".
3) The very first sign spotted in the movie when seen from above ends in the shape of a pitchfork.
4) The creatures arguably look demon like. They are not your typical "grays". They do not have small bodies and big eyes. They have super speed, jump very high, their skin automatically camouflages to anything around it, they hide well in shadows, they have partial hooves and claws, a somewhat thick carapace, and attack with a poison gas from their wrists.
5) When they first hear the creatures on the baby monitor, it sounds like a lot of moaning and wailing, along with the clicking.
6) Animals have an internal sense against these creatures, and go nuts in their mere presence.
7) The second crop sign to be seen is an above view over India, and the location of the crop signs is vaguely in the shape of an upside down cross (though this isn't a particularly strong image the way it is presented, if it is intentional).
8) The creatures demonstrate no technology whatsoever.
9) There are no pictures of ships in the sky, ONLY lights. Things die when they approach the lights. The lights are invisible during the day.
10) The lights in the sky appear overnight, and they specifically say they detected nothing coming in by radar. They just appear.
11) It is quite obvious why the creatures wear no clothing or protective gear - their primary ability is that their skin automatically camouflages, and that would be hampered by clothing.
12) The creatures only attack at night.
13) They say the "Battle turned in the middle east, three small cities there found a primative way to defeat them."
14) It is not at all clear that water is actually killing them. When water spills on them, their camoflauge ability fails in the area hit, and they seem to experience some kind of anguish and fear. But the creature is only seen to die as a result of a very strong whack upside the head by a baseball bat, and the only actual clear injury aside from the bat is from a knife cutting off its fingers. Areas hit by water don't look pretty, but it's not really clear that the area is actually injured.
They sure don't come across as aliens that travelled the galaxy in space ships using technology. Whatever they are, they are a lot more primative than your typical iconic high-tech "gray" space alien.
I think the argument can be made that these creatures are demons. Or, they are aliens. Or, they are creatures from another dimension. It is left quite up in the air. But I no longer think the folks who saw these creatures as demons or something from another dimension are as totally off base as I used to.
