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?"
The events of Paranormal Activity 2 take place before the events of Paranormal Activity (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, which is a prequel to the both of them. Paranormal Activity 4 is really the first sequel, chronologically speaking, to the original.
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.