It's a mystery (and I love a good mystery, don't I Mrs McCann), also I'm ex forces and I live in the area. I was in Bury St. Edmunds yesterday.
This report on todays BBC news pageshttp://www.bbc.co.uk/news/uk-england-suffolk-38267086.
I've spent a lot of time previously thinking along the lines of he's gone AWOL and means it, or is now hiding and scared to report back. The forces don't like AWOL, they'll send you to jail.
However, now I'm wondering.
Why didn't he crash out in his parked car instead of sleeping in a shop doorway.
Also if he went into that area behind Greggs (loading bay), was seen going in by CCTV but not coming out, what if he went to sleep inside a parked van that later left not knowing he was inside. His phone was last picked up at Barton Mills which is a large busy 5-way island at the junction of the A11, A1101 & A1065. A perfectly natural route for a delivery van.
If he was discovered at that point and kicked out someone knows this and hasn't come forward. If he woke up there and jumped out, then decided to hitch it or walk from there, given his condition (fall down drunk) he could have been hit by a car and be lying in a ditch at the side of the road (A11 or A1101?).
A bit fatalistic I know but not an impossible scenario.