The Navigo Decouverte does not run Sunday Am to Monday AM. It runs Monday to Sunday midnight. It is the best travel bargain in Europe if it works for one's trip.
I don't recall the day of week we were at Denfert Rochereau when my husband was surrounded by thieves, but we were coming in from the US, so I would guess we were there mid morning by the time we cleared immigration and got into town. We were on our way to a friend's place near Montparnasse where we were cat sitting for two weeks while they traveled. A French women did intervene and chastened the thieves who were 3 good sized men. I was carrying our heavy messenger bags with computers and such and had stupidly gotten out ahead of my husband and so he was alone on the stairs with the two suitcases. We have often had pickpocket attempts, but this is the only time it has been so brazen. Most of these are organized criminals and are skillfully trained to be undetected. Once my husband literally put his hand in his pocket and discovered the hand of a young woman was already in that pocket. These three guys were not highly skilled; they were just thugs.
Basically pickpockets are not big deal; usually you don't even see them work; you won't lose anything if you don't carry valuables in pockets or other easily accessed spots like backpacks or purses carried in back rather than cross body in front. I use a money belt in transit from the airport when I have all the valuables and passports and then stow them in hotel safe or apartment.