A hair band is such a small thing that it wouldn't have made me think there was any cheating going on in the first place. I might be curious enough to ask whose it was if you didn't live with a girl but unless there were other, much bigger things, making me worry I'd happily accept your explanation of whoever it belonged to. I wouldn't care if it was your Dad's, your sisters, a friend (of either gender) or even your dog for to be honest. It's a bobble. It wouldn't bother me.