Critical things missing from film plots are not something unique to the writers of Five-Zero.
At the end of the 1944 "classic film noir" Laura, Waldo Lydecker (Clifton Webb) comes back into Laura's apartment after McPherson the cop (Dana Andrews) has left. Lydecker, who previously left her place, enters through the kitchen door, which is beside the main door in the outer hallway, which McPherson has made sure is locked after he leaves. There is no explanation for how Lydecker has access to the kitchen door. It is suggested that he has his own key to the place (see below), but I did a search through all the subtitles in the movie, and there is no reference to him having a key, despite the fact that it is quite possible that he did have a key, because he was obsessed with Laura and was very devious in having her and her friends tailed, for example. But Laura lives in an apartment building, not a house, and it seems like a convenient coincidence that this kitchen door just happens to be where it is. Shelby (Vincent Price), Laura's off-and-on fiancée, did get a key from Laura's desk at work when he wanted to use the place on the night "Laura" was murdered to have a talk with this woman he was seeing.
But it looks like I am not the only guy to notice a "goof" in the film:
McPherson tells Laura to remain locked in her room and to open the door to no one. After he has departed, Lydecker, who had not left the building, emerges from hiding. As Laura prepares for bed, she switches on the radio to Lydecker’s evening broadcast, his subject, as he told her when departing moments earlier, is the great lovers of history. The broadcast is what was then called an electrical transcription. ... Lydecker enters the apartment with his own key, opens the clock and loads the shotgun with two shells from his pocket. He proposes to kill her, saying if he can’t have her, no one can.
Commentary on Fox DVD by Jeanine Basinger: She [Laura] turns out the lights and goes alone into her bedroom. And we see she is alone. And now swish-pan over to the kitchen door which everybody conveniently forgot, Waldo would have a key to, or would be able to enter through, and here is Waldo. [Basinger then tells about how it is "good screen writing" that Waldo has set us up prior to this to know that he will be on the radio about this time!!]