Should I be worried?! Am I over thinking?!

So today my boyfriend was getting ready to bring me home when he got up from the bed he found a chain for a necklace on the floor and handed it to me and said here thinking it was mine. I thought it was mine and that my necklace I was wearing broke off or something I touched my neck but it was still there. Well this chain is not mine and I know it for a fact because I have 3 necklaces and I know exactly where they all are. He said well its gotta be yours cause your then only girl I've had over here in the past 10 months. A part of me doesn't believe him, idk if I should or not. Why would a random chain from a girls necklace just pop up out of nowhere on the floor next to his bed. I didn't make a big deal I just put it down on his side table and continued what I was doing. Wtf!