I've thought about doing this when backpacking, but do you worry that putting a return date on your car is an invitation for thieves? Or do you just figure that any theif willing to head to a trailhead pretty mych knows that the vehicles are going to be unattended for an extended period?
As for diving, I always tell my wife where/when I'm going, and call her as soon as I'm out of the water.
From good advice on other threads here, I plan to also leave a list of safety/signalling equipment I'm carrying so rescuers know what to listen for...
When we backpacked, it was in pretty remote areas. But we also would talk with the ranger station before heading up and knew their schedule. We never had a problem with break ins, most of the time there was nothing left for anyone to take but the vehicle. Like I said, if someone wants something, nothing will stop them from taking it. From the cases of lost hikers, most of the time it was rangers who found their vehicle after a family member reported them missing. It's definitely easier for them to find you when they have your path and destination to go by. But in the end, it's what you feel most comfortable with.
Be safe!
Carolyn:sharks: