Source for this? As I recall, his phone was off when he committed the murders. In fact, they used the evidence that it had been turned off just for the duration of the murders (with some padding) against him.
If you're going to commit a crime, don't suddenly turn off your phone if you don't have a history of doing so!
I have on at least two occasions forgotten my phone in my car at highly inconvenient times... One of which was just before a flight, for a vacation. I was amazed it had any battery life at all when I got back.
Yes, this stuff does happen to real people doing normal things.
Sure, but that pattern isn't going to make the cops think you committed a crime. And if you flew out, there's plenty of other evidence you're elsewhere (airline information, credit card usage, etc).
To clarify, I'm not saying they used it against him in court (he didn't even have a trial). It did help in increasing their confidence in him as a suspect.
If you're going to commit a crime, don't suddenly turn off your phone if you don't have a history of doing so!