The problem here was the police not taking the tape into evidence. Those who have actually worked with surveillance cameras through DVR know that footage gets written over after x amount of time. The police should had either confiscated the recording or asked for a copy of the recording immediately.
I don't know if this is what happened here, but there's really nothing funky about a DVR writing new footage over the old footage in a week's time.
What happened here is police negligence if anything. If something else happened, well we really don't have the information to know. What we do know is of the bad job done by the police. But the police did watch the footage, don't know what that amounts to.