It just shows relatively how full that container is, can be helpful to indicate containers that are being used to store stuff vs the ones that are empty or not being used at maximum capacity
Dagger means about half the slots are occupied with items, dagger on purple cloth mean its filled completely.
Devs are just teasing you by showing items that are not acquirable in the game.
I'm not sure that I have a strong argument against sketch colors - Jorb, November 2019 http://i.imgur.com/CRrirds.png?1 Join the moderated unofficial discord for the game! https://discord.gg/2TAbGj2 Purus Pasta, The Best Client