
If u read it properly you'd understand why. Shinba probably knows the route tatami takes to school, so he'll definitely search the subway station. And if he finds the ring he'll know that Sou was there too n he's in trouble. shinba was the one who gave that ring to sou, he'll recognize the ring easily

I know that Shinba gave him the ring and would recognize it, but like Jam said both of them weren't where they should be. His sister was following the drug guy and his boyfriend was following his sister :D But let's say he just searched the whole area, manages to spot the small ring before anyone else... I'm curious how the mangaka will explain him finding them in the end. Maybe some cameras? Though only the police should be able to get excess to the vid material... so again, no idea :D

To Stormborn, no, actually tatami wasn't following the drug guy. When she said she spotted the "delinquent from yesterday" she meant Sou, so she was trying to take a detour from her original path down the subway when she encountered the druggie. So there is still a possibility Shinba searched the underground tunnel path bc that's tatamis everyday route.
I could be wrong and tatami doesn't use the subway everyday, tho. That means shinba would have to search everywhere around tatami's usual route and by chance he'll search the subway tunnel and find sou's ring.

Yes true, but didn't she start following the drug guy after she noticed what was up? Though it can also be seen as her just going her usual way back.
Still :D either way. A hell of a small chance Sou is setting his hopes on here :D no way that'd work in real life. But so what for the sake of them being rescued I take everything!
Btw why in god's name should Shinba notice the ring at some train station in some random corner? Why? And how should he be able to figure out what happened or where to look for him/them? :D srsly, there should be nearly no chance for him to find that ring!