If we go on the theory that it was a pre-arranged pickup, would it be safe to say that it was a taxi, a web hookup or a friend? Friend is prob ruled out as most of his buddies were out that night themselves and it sounds like he never did meet up with them. So - taxi? Or web hookup?
Earlier, we talked about Grindr and Tinder and how the info in those apps is impossible to trace without the phone itself. Would anyone know - if you logged into your account on these type of apps from a different device, does it show your history? Like, who you've talked to and maybe the message stream that went back and forth?
It sounds like it'd be a way to see if he did communicate through an app? It's only been said that he sent a photo to a "friend" at 3:08 - he could have sent the person the photo of he and Darroch so they'd recognize him when they pull up in the horseshoe area? (Like: Hey, can you send a recent photo so I really know it's you?)
Taxi seems like it'd be easy to trace (do you have uber and lyft there in the UK? - an app where anyone can use their own car as a taxi service thru the app). Could he have used something like that to get him back to base?