I've had this happen a half dozen times. It happens most often when I turn the truck off for a minute or two (e.g., run in to check the mailbox) and then back on. I always let the truck boot, but that doesn't help. It looks like a discrepency in the time-out values between the Nav and the radio.
I've found that when it happens, the Nav Screen stays on with just the green oval. If you turn the vehicle off until the Oval goes dark (3-5 minutes, not hours) then when you turn it back on it is fine. I don't have an alarm button on my fob, so I don't think that anything is needed except time (when it first happened, I thought it was because I didn't lock the doors at the post office, I started being very careful to lock up every time I got out and the problem kept coming back)
David
'05 LR3, New Mexico