It is not necessary for you to read out every single thought that comes to your mind; not because you don’t believe in him but because you wouldn’t want to hurt him.
Sample this: your husband is talking to his mom on the phone, you accidently overhear a bit, then get curious and secretly listen to what they are talking about. You heard it, you knew about the topic, and then it’s over. It does not harm them, it does not harm you, then what’s the point in confessing to him? And who knows, your husband must be harboring some secrets too!
1. An old flame turns up:
Perhaps in your thoughts, as you recall him. Or you indeed bumped into him somewhere. It was sort of rekindling, but you could do nothing about it (or did you?). Once home, you don’t discuss him but have fleeting thoughts about the meet. You don’t have to tell your husband about him. Why would you?