Just began reviewing this case. Pardon me if I speak things already discussed.
I just read ". . . They've continued to call her cell phone hoping for a response.
All they hear is: "Sorry, that mailbox is full. Please call again later.". . ."
http://www.cnn.com/2007/US/06/15/florida.missing.woman/index.html
Not a cellphone expert but I wonder if L.E. can get phone service provider to delete the family messages off the server and if that would in turn show up in the phone or if the individual phone must also delete them from its memory. It might depend on where the messages are stored prior to reading the messages. If the messages are stored on the company's servers and if the little text notification is all thats on the phone then that list might update if the company server deleted the stored messages.
But if the phone has to actually clear its memory by the owner hitting a delete key then they will have to try other things. Perhaps the phone will send out a ping once in awhile if it is powered up and the towers can be used to get a fix on the location.
In any case the phone should have been traced to where it was last turned off. Surprised if that has not yet been done.
If the family acquaintance (probable POI in my opinion) borrowed the car then where did he return it? More importantly, if he lied about borrwing the car how did he get home from where he buried/hid/drowned the car?
Do we need to look for a brother or close friend that would have given a poor guy a ride after a tiring night's kill?
Reading between the lines I get the idea that L.E. does not believe the family acquaintances story and suspects the car will hold proof of a crime or else it would have showed up to be seen by all.