The pocket item you are looking for is connected to the green apple quest. Try inspecting the postcard you received, it should give you a hint about what to do next.
Thanks a lot ! I have finished the game at 100% now.
I just want to add a suggestion why not adding a phone number with the key on the locker next to the entrance of the shop like you only put 4 keys to make a phone number with the numbers on the keys.
