The events of Paranormal Activity 2 take place before the events of Paranormal Activity (2007) (beginning two months before and following up with the events depicted in the original film) which technically makes it a prequel. Such is true with Paranormal Activity 3 (2011), which is a prequel to the both of them. Paranormal Activity 4 (2012) is really the first sequel, chronologically speaking, to the original.
On the official site for the film, there are secret messages when the trailer is fast-forwarded or rewound. Additionally, after the trailer finishes playing, the baby appears in the mirror but isn't present in the nursery. Also in the mirror, letters written in what seems to be blood spell a message. Upon flipping the picture horizontally, the message reads, "What is happening to Hunter?"
Upon release, this broke the record for the biggest midnight gross of an R-rated movie with $6.3 million, and the biggest opening for a horror movie of all time, earning a total of $41,500,000 in its opening weekend.
(at around 1h 16 mins) During the climax, immediately following when the power goes out in the house, there is a single, out of focus, frame of Hunter crawling in the street that quickly flashes on and off screen.
The trivia items below may give away important plot points.
Just like in the first Paranormal Activity, it has been confirmed that there is an alternate ending to this film as well; where Katie kills the baby boy Hunter by breaking his neck. According to the website Bog Bit, it was never shown to the public eye because it had been claimed as "too violent." They also quote: "For those die hard Paranormal Activity fans, they may be dying to see the alternate ending as well as looking towards a sequel, but this new ending is going to push a lot of viewers away."