I know my daughter's fiancé and his roommates had a horrible habit of leaving a backdoor of their house unlocked all the time. They were living on Orange St. at the time (the other end, not at the apartments). They only stopped AFTER the murder. Now, they have moved. Young people are too trusting. 10ft tall and bulletproof thing, I guess.
As for master keys, they don't look incredibly different. I was accidentally given a master key to my office a few years back. I only discovered it opened other doors by accident. A coworker forgot her key and we decided to see if anyone else's key might work. Mine did. Out of curiosity, I tried another locked empty office. It worked. I then became a secret squirrel with a spare key for anyone who needed a door opened - mostly because management was always late. We never told management about the key, and when I left the position, I passed the key on to another coworker. It happens.
I've been respsonsible for master keys many times over the years for various reasons. A few looked "incredibly" different; most were "distinctly" different.
Plus, from a money standpoint, getting 20 or 30 of the same key copied is a little cheaper than getting 20 or 30 different keys made. If no one is told their key will open up other apartments, they may never know.
Speaking of distinctions, the hearing testimony and the warrant draw one between the master key and the key to LG's apartment. But, wow, what a liablity it would create to give all the residents the same keys.
I can also see how LG might have given McD a copy of her key for when she was out of town, whether in Atlanta with her boyfriend, camping with friends, or going home for a visit. He was convenient to take care of little things like her plants or maybe feeding the dog on her short getaways. Yet, with a key outside the door, it seems redundant. Maybe she removed that key when she went out of town. Who knows? But, he can always claim that and it is a reasonable possibility. And can lead to doubt about the key being part of a sinister plan to murder.
How McD acquired the key will likely be an issue only if the DA builds it into his argument. I don't think he will.
Just tossing out multiple random thoughts as the coffee sinks in this morning.