My real question is, what was he doing in the truck between 03:48 when he gets in and 04:32 when he puts his phone into airplane mode?
OK - i think i see where you are going with this, and it potentially disastrous for the defence!
I think BM was messing around with SMs phone (deleting messages?) and maybe then staging it. There is no other reasonable explanation.
Recapping key times to avoid need to cross reference.
03:48 - gets into Truck. Door does not open until he is in Broomfield - so it is a reasonable inference that he is in the truck for the next 4 hrs and 22 mins
03:58 - the truck is moving west - in the direction the bike/helmet was found
04:10 - 04:23 - last activity on SM phone
04:32 - BM phone into airplane mode
05:14 & 05:16 - BM truck on traffic cams heading to Broomfield
It is surely too much of a coincidence that the last activity on SM's phone is just 9 mins before BMs phone also goes dark. Are we expected to believe that having no real signs of digital life since sunbathing on Saturday, suddenly there she is at 04:23 am turning her phone off - only never to turn it back on again despite BM being away? No check for messages from her AP? Not credible.
As BM does not get out of his truck until 8.10am the only rational explanation is that he had SM's phone in the truck with him. He has to be the person who turns it off.
We know he parked up (he tells us this).
So my guess is he may have been deleting messages or conducting one last sweep of the phone (this could be why it shows activity 04:10 - 04:23 but we have to wait for the evidence). My guess is at 04:23 he turned the phone off and threw it out the window (or binned it in broomfield).
Until 04:32 he may be cleaning some of her messages off his phone??
I wonder if he then realised he blundered having his phone on out there - hence going to airplane, because there is no reason for him to have his phone in airplane mode on the drive ....
This sinks the defence IMO.