Must have been something
Jenny is alone at home. She hears a loud BANG! What happened?
It must have been her cat!
You left a slice of chocolate cake on the counter 10 minutes ago. You went to the bathroom and when you came back it was not there anymore!
It must have been my sibling!
Your friend, who never gets bellow 90% on his grades, just flunked geography.
He must have had problems at home.