Cover of the day - Detained in France

[Jim Etherington]

This cover had an interesting journey with an unexplained interlude. Sent from Margate, where the 2nd Echelon BEF was based, on 7 May 1940 to Major A E Taylor Royal Tank Regt., No 1(?) General Hospital, it was redirected to No 7 General Hospital (possibly on 10 May – manuscript addition). Back-stamped with FPO 37 (18 May 1940), Army Post Office (19 May 1940) cancels it was returned to the UK (London SW1 - 29 May 1940)

The next dated cancel is Home Post Centre  ? May 1945 where presumably the 'DETAINED IN FRANCE / DURING GERMAN / OCCUPATION' cachet was applied. Detained mail was returned to the UK from Paris during April-May 1945.

It was then forwarded to DDMS (Deputy Director of Medical Services), British Troops in Egypt, the cover being back-stamped with a FPO 76 receiving office dated 18 May 1945. As a consequence of Taylor not being found, the cover was returned to the UK (London SW1 1 June 1945), with 'Not known  M Branch BTE (British Troops Egypt) 18/5/45 affixed. Presumably it was then forwarded to Weston Super Mare, but there are no postal markings to confirm this.

How did this cover come to be in the French civilian postal service and end up in the vault of the Paris Post Office for the duration of the war, having apparently been returned the UK by 29 May 1940?  Any thoughts?