My theories at this point, in no particular order:
1. Grindr or rough trade hookup gone bad
2. Unlicensed taxi or stranger offering a lift (both with criminal/murderous intent from the get-go)
3. Unlicensed taxi or stranger offering a lift, drives to remote area to put the moves on Corrie, Corrie's not into it, driver flies into a rage and kills him (someone on one of the earlier threads said when he was younger in the same area I believe, he accepted a late night ride from a random guy. The guy drove him into the woods to get down to business, but when rebuffed, luckily let our poster go.)
4. Accident (which could range from the lorry driver hitting Corrie if the lorry driver had been drinking, to Corrie OD'ing in a sketchy house, to him somehow making his way out of Bury on foot and being hit by a car on the road walking back to base, or slipping and drowning in a body of water)
5. Crime of opportunity terrorist abduction (could also be posing as unlicensed taxi or Grindr hookup, just to speculate)
6. Involvement of another service person up in Mildenhall.
7. Suicide or AWOL - I think he shows strong signs of being bi-polar / manic depressive. Technically, schizophrenia hits some men very badly at about this age, even when they've shown no signs before. Admittedly, this is lowest on my list, but I do think he may be manic depressive and have impulse control problems. His "reckless" behavior in the past would fit manic episodes.
8. Same guy abducted Corrie that abducted Luke Durbin and has never been caught.