Okay, here we are: Deb comes in to see LaGuerta, having been summoned, and LaGuerta hits her keeping-her-job talking points, saying that she was in fact overzealous because of her relationship with Doakes, and what she put Dexter through was negligent and unforgivable. I was thinking when she was saying this the first time that she was doing a good job of selling this; little did I know just how good.
Deb sits silently as LaGuerta goes on that her career is going to take a hit, and Deb at least has to express that she hopes LaGuerta doesn't lose her job. LaGuerta thanks her for the thought, but goes on that if that does happen, she'd like to leave things in good order, and she was so busy with the BHB angle that she overlooked some loose ends on the Colin Hanks death, so she was hoping Deb could help her with the timeline. Deb, who really looks a wreck, not surprising given the emotional and Xanax abuse she's been through lately, agrees, so LaGuerta gets some facts about Dexter's final forensics sweep.
When Deb says that she wasn't present, as Dexter doesn't need supervision, LaGuerta's tone and manner completely change, as she's like, oh, how very interesting -- in going through my mail that was piled up waist-deep in my office, I came across a package from Anderson's widow. It turns out (presumably after the fire), Anderson ordered security footage from all the gas stations near the church, which is good police work you'd think someone else on the force would have replicated. When he died, somehow the footage got lumped in with his personal belongings, but his widow eventually came across it and sent it to LaGuerta.
Deb is tensing for what she knows is coming as LaGuerta pops the DVD into her computer's drive, and sure enough, clear as day, there's Deb filling up a can of gas on the night the church burned, with a time stamp of only 22 minutes before the FD was called. Deb does her best to talk her way out of this one, saying that she forgot that she brought Dexter some food, but the proximity of this event to the fire would alone make it almost impossible for anyone but Deb or Dexter to have set it, and that doesn't even take into account the fact that Deb is putting gasoline into a container instead of her car. LaGuerta closes the computer and asks if there's something Deb wants to get off her chest, and considering she must now know that Deb probably actively thwarted her investigation into the BHB, it's hard to begrudge LaGuerta a little attitude here.