Shanks was preparing to leave Elbaf when Kid showed up. From beating Kid, he got 2 RP rubbings (Wano and WCI), and now he just needs the Zou RP and the RP possessed by the Burned Scar Man. Shanks is one of the very few non-Minks in the world who knows that Zou has a RP (even BB didn’t know Zou had one), and that’s because he was there when Roger went to Zou for the RP. So it’d make sense that Shanks’s next course of action is to go get a copy of the Zou RP, and this will be a convenient excuse for Oda either keeping Shanks out of the Elbaf arc or having him show up at the end of it