If its alarmed and you're close enough to hear the alarm and get there within a matter of seconds (quite likely in that situation) I think you could make a pretty good argument that it is supervised even if you're not in direct sight of it.
If I'm travelling longer distance with guns in the car I have them bolts out and locked to a luggage tie-down loop or something with an alarmed cable lock (as well as being out of sight), and obviously lock the car if I leave it to go to the dunny or buy food etc. If anything did happen at least I could show that I'd taken reasonable steps to secure them.
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