Not really, a envolope on your drivers seat isn't going to temp a theif anymore then a vehicle in a trailhead parking lot. Having the information in your vehicle is a goos idea, if you are reported missing and your vehicle is found PD will be able to gain access and that info (including what excat time you left your truck) will be beneficial to searchers. Having a friend who knows, and has that information is good... but duplication is better! You know, one is none, two is one.

And, you leave you vehicle in a trailhead parking lot overnight and it IS going to get broken into... it is just a matter of time.

Like Doug, I am a firm believer in PLBs however it is not a replacment for notifiying a friend.