TIFU by assuming a customer was returning something

Ok, so I work at a department store in a mall. I’ve only been working there for 2~ish months and am still getting used to the flow of retail since I’ve only ever worked fast food. Earlier today, I left my register for a few minutes and when I came back there was a paper bag of clothes next to the register. I didn’t know why it was there, but I kinda assumed it was a return since the bag wasn’t from our store and was crinkled all to fuck. Maybe someone returned it while I was away and my coworker just left it there? I needed to get it out of the way though, so I put the clothes under my desk and threw the bag away. Soon thereafter, an old man comes up to my register asking where his bag is. I grab the clothes and show them to him, now assuming he wants to return the clothes with me. He asks where the bag is so I fish it out of the (clean) trashcan. I ask for proof of purchase, like I always do for returns, and he looks at me like I just called him a slur. He says something about the bag so I look in the paper bag to see if the receipt is in there and I just didn’t see it. He starts loudly saying how rude all the cashiers here are and how cashiers shouldn’t ignore customers when they’re needed (not sure where that part came from). He accuses me of thinking he’s a shoplifter, and I say no, I just need to scan his receipt. He angrily fishes the receipt out of his wallet and I try to scan it, only to realize that the receipt isn’t from our store. So I think “oh, this dumbass is trying to return something that he didn’t even get from here. Wouldn’t be the first time”. I point out calmly that the receipt isn’t from our store, and he says “yeah of course it isn’t, I didn’t get it from here.” I tell him I can’t return it for him if he didn’t purchase it from our company, because I know customers who would need that explained for them. He says something else rude, takes back his clothes bag and receipt, and smugly says that I can go ahead and call the police on him if I want before leaving.

I didn’t realize until like an hour later that he had probably just bought that from another store in the mall, left the bag of purchased items on my desk while he did something else, and just wanted his stuff back. He had no idea why I was asking for his receipt because I never asked “are you returning this?” I just… assumed. Whoops. Call it the tism if you want.

TLDR; a customer left his stuff on my desk, I assumed he was trying to return it, and he thought I was accusing him of shoplifting.