The thing about the DJI Mavic drones is they actually have extremely limited battery life. From personal experience, you’re lucky to get 18-20 minutes total flight time out of them despite the advertised 31 minutes that the manufacturer claims (presuming Russell has the Mavic 2 Pro model).
It usually feels like you’re barely airborne before the return-to-base battery warning kicks in (I think it’s at 30% juice left, by default) so their range is not particularly huge especially if you’re just having a nosey around with no fixed, pre determined destination in mind. It’s very easy to misjudge the amount of juice left and therefore if you take it too far you have to bring it down (or it auto lands itself) before you actually get it safely back to the “home” starting point. New users are usually quite reluctant to take them too greater distance until they’ve got a real handle on the speed of the battery drain, and imo the limited battery life it is one of the biggest surprises (disappointments) a new purchaser gets after buying one of these - it’s a real big limitation.
Chances are the range of that model drone that Russell owned could hold the key. Plotting a radial search area from the campsite using different battery life/warning alert/speed scenarios, and then taking the search into the most difficult/scenic spots would be my focus. I’d run some scenario flights from the campsite using exactly the same model of a similar age. I think the probability is high that the drone ran out of juice before getting back to base, the couple went on a retrieval expedition no more than a few kms away, but that terrain proved much more difficult than anticipated.
Obviously, police will also be looking at which device (mobile phone or tablet) Russell usually had connected to the RC controller used to fly the drone and which has the DJI “Go” app installed on it. If that turns out to be the mobile device left still charging at the campsite, that tends to rule out a drone loss/recovery being the cause of their disappearance.
