One thing that I've noticed, going through returns for the British in the Peninsula, is a steady trickle of men entered in the "joined" columns as "returned from desertion". I guess that a fair few of these would have been dragged in kicking and screaming by the Provosts, but, equally, I suspect that a lot of them are returned stragglers. I've been producing some graphs to track desertion across the conflict and, as one might expect, there are substantial peaks every time the army makes a major move. This, to me, says stragglers falling out on the march, rather than deserters proper - ie, leaving with no intent of coming back - but that they were being struck off the lists somewhat prematurely. This was particularly endemic in the 5/60th in the first year of the war, which ends up having to list well into double figures as "returned from desertion" once it finally gets its act together.
Frequently, in the returns, there are notes explaining the circumstances: the most extreme case that I've found was a bugler from the 95th, who was cut off during the retreat to Corunna, spent an unspecified time in French hands as a POW, and eventually turned up at Gibraltar in spring of 1810. There were a couple of others from peninsula regiments who turned up at Stralsund in 1813, but I assume that they had been POW in France or Germany.
Don't know if this helps much in the general scheme of things, but its something I've been speculating on as part of my research, and I too would be interested in anyone else's thoughts on the matter.
Best regards, and wishing a Happy New Year to all, Andrew