Yeah, about leaving the iPhone sitting on a table for about an hour and coming back and finding it had been reconfigured...
Music library is very dependant on the synchronisation settings you have on the PC you use to back up the phone. The PC has to be authorised by the phone user and linked to the same Apple account as the phone. There are certain steps you need to take to transfer across stuff you have backed up from an old phone to a new one. Possibly the PC was still set to the old phone settings. Anyway, you should still be able to recover all your iTunes content by downloading again from the iTunes store. A faff, but these things happen.
Bluetooth problems with certain cars is not limited to iPhone. Not all vehicle manufacturers follow Bluetooth protocols to the letter, although Apple products can be a bit picky about not cooperating with devices that are not 100% compliant with the protocols. There may be a software update available for your car, worth checking with the dealer.
Never, ever, ever, ever, leave your phone unattended if there is the remotest possibility that someone else might have access to it, and not even then! Even when a phone is locked, there are certain exploits/practical jokes that can be performed that only require lockscreen access. Just a touch to power the phone up and make a connection is all it takes. I will not be providing further information about this, but I'm sure there are plenty of dodgy Internet sites that will.