I think, maybe, the person who contacted her relatives/friends is someone who is trying to protect Fitz's situation (if needed) a kind of pre-emptive help. What I mean is...y'know when someone *advertiser censored**s up and they get a friend in to help cover up the situation...well I just think someone is trying to help him but maybe unaware of what has happened.
It's obviously one of 3 things, one being accident/car breakdown in dessert and they succumbed to the elements...they are both no longer with us. An ex-Seal who I briefly chatted with said Fitz was very capable with survival skills
2. another option could be for a number of reasons, things between them took a turn for the worse and, well...he caused her demise
3. someone/gang, for whatever reason, vehicle hijack, robbery - left them for dead
I just don't get why there's a thought that they went intentionally missing together. My friends & family know I'd NEVER do that, just as Jin's family know that she wouldn't