Returning my book to the library

Went to the library to return three books. One of the books I had checked out never had registered on my online library account, so I went to the circulation desk to let them know the issue. Two of the librarians went to scan it back in, and found no library barcode. They looked at me and remarked, "This book isn't ours."

I stared at them, and then down at the book, and then said confused, "Huh...I must've bought this one then."

They laughed hard at me,"Sounds like you've been studying too hard!"

I chuckled a little too, and said,"I really don't remember buying this book."

"Well it was nice of you to try and give it to us!", and they continued laughing.

I put the book back in my bag.

They were still laughing as I walked out of the library.