You have a detailed story on the following link in French:
http://dkepaves.free.fr/download/chronologie.pdf
To sum it up, the French plan was to send a French Army to SW Netherlands. A part of the troops crossed Belgium by road, and some troops were to be carried by ship to Vlissingen. The first convoy left in the afternoon of 10 May and arrived in the evening.
According to the above source, the sortie where a German aircraft was claimed by de Scitivaux was an escort mission for Laté 298 floatplanes fying recon of the Scheldt mouth.
According to Arnaud Gillet's book "Les victoires de l'aviation de chasse française, vol 1, 10-15 mai 1940", 4 Potez 631 of escadrille AC1 based at Calais-Marck were escorting one Laté 298 of T3. EV1 de Scitivaux was late and flew alone. He possibly reached the recon area, did not find the formation and with available gas and nothing to protect decided to fly a little more north.
By the way, the Potez 631 of AC1 also flew cover sorties in late afternoon over the convoy going to Vlissingen.