Throughout the series police officers are said to have transferred from other departments and even other states. This is not a thing. Florida law does not allow this. Even inside the state a police officer must quit his job and be rehired by another agency.
(at around 26 mins) Dexter spots plastic in the window of room #129. This tips him off that this is Zack's "kill room". Dexter picks the lock and as he walks in we see that there is plastic covering the entire room. First, there would not likely be plastic between the blinds and the window as we first see, as it would 1: draw attention and 2: the blood spatter would hit the blinds on the inside, not the plastic. Then as Dexter opens the door, we see there is also plastic sheeting on the inside of the blinds that would prevent blood spatter from hitting blinds but also makes plastic behind blinds unnecessary, except for tipping Dexter off that this is the "kill room" of course.