It's just a trash can; you'll likely have seen this message in during the intro in the city of Lumiere. After finding Sophie [[link]] as Gustave, [[link]] you can explore and examine to your heart's content, not limited to sticking your nose into the city's various trash receptacles.
Every time you try to examine a bin, you'll get a message professing its innocence, but seasoned gamers know the truth: a trash can is never just a trash can. If you've already carried on to the expedition festival, then you might want to know the answers to instead. Otherwise, here's how to unravel the mystery of the Clair Obscur: Expedition 33 trash can.
How to use the trash can in Clair Obscur: Expedition 33
After you meet Sophie as Gustave and start exploring the city of Lumiere, you'll find a number of trash cans you can interact with, but upon examining each, it'll either say "It's just a trash can" or "A normal trash can". Obviously, this is a straight-up lie.
You can uncover the mystery of the trash can when you pass the main square and enter the area with the painter and the woman playing guitar on stage. Just behind the painter and his blank canvas, you'll find a trash can with a slightly different message: "A perfectly normal trash can". Keep interacting with this new trash can for "It is indeed a perfectly normal trash can" and then "Once again, a perfectly normal-".
At this point, Trash-can Man will interrupt (yes, that is genuinely his name) and tell you that he's hiding in there so the paintress won't find him—genius idea, pal. Keep talking to him and [[link]] he'll explain there's something digging into him, before handing you a Chroma Catalyst for upgrading weapons as a reward. Congratulations; you've resolved the trash can mystery.
Also, quick note: if you find yourself back in Lumiere, be sure to check this trash can again.
: Conquer the continent
: Runaway kids
: Beat the buskers
: What it opens
: Where to use them