In the 4th movie, about 1/3 a way through a bunch of cars show up at the house across the street (the one where Robbie the possessed boy lives at) with a bunch of ladies getting out all wearing black robes, it's hard to see them at first though. I'm starting to believe they are the ones at the end of the 3rd movie and they are possibly part of the old cult that this symbol represents.
But these movies brought up a lot of questions to me. So this is where I will record my answers!
The Reoccurring Symbol: What does it mean?
Observation: In the 4th movie, about 1/3 a way through a bunch of cars show up at the house across the street (the one where Robbie the possessed boy lives at) with a bunch of ladies getting out all wearing black robes, it's hard to see them at first though. I'm starting to believe they are the ones at the end of the 3rd movie and they are possibly part of the old cult that this symbol represents.
They say in the third movie the symbol is used by a covenant of witches in the 1930s. Once the girls were old enough to have children, they would become pregnant and their sons would be taken at birth. The girls would then be brainwashed into forgetting their experiences with the covenant.
But is it real or fake?
I Googled "covenant of witches in the 1930s" and something called The Massacre of Innocence came up a few times. Pretty much in biblical times you were a witch if you did anything that counted as child sacrifice such as abortion, and not for practicing witch craft. But the results said nothing about this symbol.
My research on this topic will continue in the future.
So I found a similar symbol recently. It's the same exact triangle and circle, except it's got a line down the middle. It's the Deathly Hollows symbol from Harry Potter! But the one shown in the Paranormal Activity movies doesn't have a line so I think it's may be inspired by the symbol in Harry Potter possibly.
Resources
http://paranormalactivity.wikia.com/wiki/Paranormal_Activity_3
http://www.theforbiddenknowledge.com/hardtruth/massacre_of_innocence.htm