Someone else's signature appeared in my signature list in Preview app?

A signature of someone, who I've never heard of, appeared in the signature list of my Preview app. I have noticed that the signature is stored in keychain and can be found in the KeyChain Access app. I don't think I have login my Apple ID on other machines, so it's unlikely that it's synced via the iCloud. I have also checked the devices under my iCloud account, nothing suspicious. Maybe someone has touched my Macbook? But it's not very sensible for someone I don't know to sneak into my home just to create a signature using my password protected Macbook but did not do any other harm, at least to my knowledge, to my other belongings. What could be the possible reasons?