I think my dad is up to something. Should I investigate?

OK so I really have no one else to turn to other than you jellies, since you are the most detached to my family and are the most guaranteed to be able to give the objective answers.

A few hours ago my brother and I were sitting in the dining room minding our own business while waiting for everyone to bring their plates to eat together. Mom and dad were in the kitchen next to the dining room. Suddenly my mom’s phone rang (it was in the kitchen). My brother came to fetch the phone but he was too late. Mom then asked him to see who had called. He unlocked the phone and saw that mom had several missed calls and text messages (she doesn’t have the habit of checking her messages regularly). Out of curiosity, my brother opened the message box and saw, among some boring text messages of mom’s colleagues, a seemingly out of place text messages of a woman. Her number isn’t saved in mom’s phone. He was puzzled and asked me to read them. It turned out to be a hateful curse of a woman accusing someone of being selfish.

The whole text conversation was like this: there was only two messages of the woman, from yesterday and two days ago. Two days ago she basically asked her “darling” how he was doing and if he was angry with her. She asked him to forgive her even though there was no more love between them, as she was guilty with him and the “children”. And yesterday the woman cursed her “darling”, accused him of being selfish and she and her “child” would not accept him anymore. The messages immediately reminded me of some time ago when I happened to see dad using mom’s phone alone. I was so shocked that during dinner the only thing in my mind was the messages’ meaning. I have deleted the messages in case mom sees them, but not before taking some photos of them with my phone.

There are a couple of things that bug me. Was that message directed to dad or was it just a wrong number? What exactly was dad doing with mom’s phone? Who were the “children” and “child” the woman was talking about? I’m fearing that my theory is true and what may happen in the future. Right now nobody knows about it except me (and my brother, but he isn’t old enough to understand what is going on).

I consider checking dad’s phone, but dad always brings his phone with him, and I don’t want to violate his privacy. But I also fear the future consequence. I don’t know who that woman is, all I have is her phone number. So my question is: should I let things slide, or should I investigate?