iCloud Keychain verification is for the password saving feature in Safari and the like.
You don't use that? It's under Settings -> iCloud -> Keychain.
Usually I've seen it when setting up a new device, I can have it send a verification code. It's possible someone put your number in as their verification number, but I'd think they'd have to verify it the first time.
It may be nothing, but if I were you I'd change my Apple ID password, just in case.
arn