The pier. It would have been an easily washable place, and they then used the lobster trap to move the body pieces to the daycare. Which would explain the drag marks, the minimal blood left at the entrance, and why a sword was hidden at the pier.

Maybe Keito killed Sampo at the market--he was certainly tall enough to have managed carrying him even if he wasn't used to that kind of work-- and Keito's killer was the one who Sampo left a message to meet with at the pier. The last part is circumstantial as of now, but it would make some of the oddities line up.