Just before the climax of The Departed, Billy comes to Madolyn's apartment and gives her a package to keep safe and asks her to open it if he's dead or if he tells her to.
In a later scene, we see her naming the package Costigan and putting it in her drawer in her office.
This scene than cuts to Madolyn telling Collin about her pregnancy.
What's in that package? It is never mentioned in the movie again, is it?
When Sullivan comes home he has pastries. He is expecting the woman to be there like earlier in the film.
She isn't there because the envelope contained a full account of the truth. It told her to find Dignam (Wahlberg) and explain to him the truth. He had no way to know that Sullivan was the rat since Sullivan blamed this on the other rat and recommended Di Caprio for the medal. On receipt of this information he and the lady decide to kill him rather then go to the police.
This is how he is in the apartment as she gave him a key and why she wasn't there.
The envelope's contents are never definitively explained in the film. However, it seems logical to assume it contains either evidence or at least an explanation of the truth - that Leo DiCaprio (i.e. Costigan) is the real good guy and Matt Damon (i.e. Sullivan) is the real bad guy.
Earlier in the film, Sullivan's girlfriend Madolyn finds out the truth about Sullivan through a CD - but that wasn't meant for her, but rather to show Sullivan that evidence existed against him. It's possible that this same evidence was given by Costigan to Madolyn (possibly with even more evidence/detail to be used in the event of Costigan's death).
In fact, given how the film ends, it's not too much of a stretch to believe that Wahlberg (i.e. Dignam) was either given a copy of the same evidence by Costigan or was given the evidence directly by Sullivan's (now ex) girlfriend Madolyn, which gave him the final "push" required to kill him.
The package contained the full truth about himself, being undercover and what not. To contact digman, showing the whole entire truth about Collin, everything. So she would know everything. It's very simple to figure what's in the envelope. Like he says when she is walking into her office "don't open this unless something happens or I tell you too" so she knew everything. That's why when she starts to say something, and he interrupts her saying "Whatever you're about to say, think about it real hard and in two weeks if you feel the same..." giving it time so he could do what he had to.
Are you referring to the package containing the recordings? If so, this is the evidence that instigates the 'rooftop meeting'...
Later, [Madolyn] discovers a package from Costigan containing a CD with recordings of Costello's conversations with Sullivan. Sullivan walks in as she is listening and tries unsuccessfully to assuage her suspicions. He contacts Costigan, who reveals that Costello recorded every conversation he had with Sullivan. Costello's attorney left Costigan in possession of the recordings and he intends to implicate Sullivan. They agree to meet at the building where Queenan died.
It's actually most logical to believe that it's something related to who the real father of her baby is.
Sullivan possibly has ED or is sterile (hence the irony of Baldwin saying that at the driving range). there's actually a scene where Madolyn says it's okay, it happens to a lot of guys.
Since she and Costigan did actually hook up, she now knows that it's probable that the baby is Costigan's. This is reinforced after the funeral where he asks about the baby and she just walks past him.
THAT is the most likely answer.
Any further input about this theory is encouraged.
All the paternal stuff went unsaid throughout the movie for only Vera, Damon and the viewers to stew over. Leo knew nothing of it or the identity of her boyfriend for that matter. He didn't even know she was pregnant. That is what she was probably going to tell him....."I'm pregnant to my boyfriend" (so she hopes!). The envelope given to Vera by Leo had to contain the truth about the entire story and instructions to contact Marky Mark. Wahlberg and Vera worked together to set up the hit in the apartment. She went on to give birth to Leo's son and most likely spanked by Dirk Digler.
It wasn't her apartment it was his. She knew the baby was leos. She was going to say something about it, and also that she wanted to be with him. The envelope contained the whole truth which she gave to marky mark by instruction from leo contained in the envelope which is why mark kills him in the end.
