You really would have thought that they would have caught on now that theres a race on and lots of people who need fresh supplies on a Sunday!!
You might, but there is a law in France preventing supermarkets over a certain size opening on Sundays with the exception of the last three weekends before Christmas.
Marche Plus (see Stu's useful maps etc.) is open on Sundays 9:00am to 1:00pm
The bakeries and little food shops - butchers, charcutiers etc are open in Arnage on Sunday morning too.