I tend to support Martin in his theory that effectively only one aircraft was shot down at Arques/St Omer on the 23rd: the SM73 OO-AGS.
This also explains the conflicting info in different sources where the Belgian crew members are sometimes linked to the SM73 and sometimes to the DC3. Due to the confusion of the day aircraft identities got mixed, but only the SM73 seems a confirmed loss. The mix of 271 and 24 Sqn passengers and crew probably also did not help.
My interpretation is then that all the pictures at the beginning of this threat are then indeed of one single aircraft: the DC3 OO-AUI. This aircraft really looks to have stayed behind completely undamaged on an airfield. My guess is still Merville. Alternatively it made a proper emergency landing on another large and deserted airfield. There it was found by the Germans. What is then surprising is that it later was burned, whereas the Germans were pretty eager in using any flyworthy aircraft they captured in the Luftwaffe or Lufthansa. So an act of sabotage? The alternative of strafing by the RAF as suggested is of course also possible.
The last question is then how and why apparently so many aircraft were captured in Algiers. Further searching in Sabena sources reveals, however, that after many of the aircraft got returned to them by May 31, airline services were resumed in Africa. Main hubs were Oran and Algiers in the north, connecting to especially the Belgian colonies Congo and Ruanda. On June 25, when the Vichy-German armistice was signed, the Vichy government used the opportunity to confiscate all foreign/allied airliners on their territory, mainly at Algiers. That's how the DC3 OO-AUH and several SM73's ended up in Vichy hands and were then apparently given to Italy.
Furthermore, to clarify the confusion on how many SM73's participated on the 23rd:
http://aerobernie.ae.funpic.de/Sabena.html
lists only two SM73's lost on the 23rd of May: the AGS shot down and the AGZ abandoned. The AGX and AGY listed in Paul's overview as also lost that day were in fact amongst those captured at Algiers and ended up in Italian service. They were written of in November in Brindisi.
Regads, Pieter