a very good account of what a POW went through comes fromn the diary of Sergeant Nicol of the 92nd Gordon Highlanders, recently republished by Leonaur. This will answer many of your questions re: a POW's life.
Normally, POWs were stagemarched into France proper, and whilst in France, moved about between various towns that had or had not the facilities to house POWs. There were internment camps, but often soldiers were pressed into French service, or put to work on public works. When paroled and/or exchanged, an obligation could be put on the exchanged prisoners not to serve in a certain theatre of war for a certain amount of time. Thus, your fellow could be released, yet not be able to rejoin his regiment if it was still fighting in that specific theatre of war. He would then probably be sent to his Regiment's depot, and remain with it until released from his obligation, and able to rejoin his Regiment in the field. I cannot account for your fellow not appearing on his Regiment's books for any amount of time, since POWs would continue to be carried on his regiment's books until released or confirmed dead, and would still draw pay through financial agents abroad.