What a terribly difficult spot you’re in! This problem properly belongs between your parents but without knowing your family, it’s hard to know what to tell you. If your dad is a kind and gentle man and doesn’t have a bad temper, you could tell him what you saw and just ask him what it means. It just might be that you misinterpreted something. If not and he is doing something he shouldn’t, you can tell him how much it bothers you and that he has to talk to your mother and get you out of the middle.
If your dad has a bad temper, it would be better for you not to talk to him directly. When people with a bad temper are caught doing something wrong, they get defensive and angry and sometimes take it out on the person who caught them. In that case, I would encourage you to tell your mom that you accidentally saw something that bothers you but you don’t know for sure what it means. If you think neither one of them would listen to you respectfully, maybe there is another adult you trust that you could talk with who could give you guidance about what to do. Hopefully, you won’t have to hold onto a secret that worries and maybe frightens you.
You are probably already finding out that secrets like this have a way of making relationships get strained and strange. My guess is that you don’t like how you are feeling about your dad right now and that makes it hard to be relaxed with him. That’s why I really, really hope you find a way to confide in an adult so that the grown ups will take care of it. One last thing: Please remember if your parents have a fight about this that the fight isn’t your fault. If they fight it is because there is something they need to work out together. You are only the worried kid who needs her parents to take care of parent business.
I wish you well.