Dad(52M) sent flirty messages to someone who is not my mom(50M)

I (22F) found flirty messages from my dad(52M) to another women that is not my mom(50F) and now I don’t know if I should do something.

English is not my language and this is also the first time I’m posting on Reddit so please be kind.

For context my mom (50F) and my dad (52M) have been married for 25 years. They have a very good almost untrue marriage. They are loving and caring to each other. You get the point. I am also a single chid and still in their house.

The person he flirts with is a massage therapist that also treated me and my mom for quite some time. We used to talk about all sorts of things in my consultations. She also used to train CrossFit with us. And also we’ve been to her birthday as a family. Not sure how old she is but I believe somewhere around 35-40F.

I first found these kind of messages last year on my dad’s phone when I was doing something he asked me (don’t remember what). It’s not uncommon for us to borrow our phones for favors so no big deal about that.
I saw a notification from her popping up and out of curiosity clicked on it. Started reading it and I realized they actually talked like friends and for a split second I was actually happy that they had a close relationship. Then I read some more and saw some flirting that made my stomach drop. Nothing graphic. Just flirting that felt like it caused some tension iykwim.

I felt this panic and anxiety like I was the one being cheated on. Didn’t say nothing and somehow manage to calm down and tell myself it was just some nonsense flirting.

Today I was on his computer filling some paperwork and his WhatsApp was logged in and open. I hadn’t forgotten what I read so I went to check the messages again and still found some flirting but this time it felt way worse.

For context, we visite Scotland as a family this summer and my dad bought a kilt. He mentioned it to her and made sure to clarify that you wear nothing underneath. Then he said that he would wear it if she wore her lab coat (for lack of better translation) with nothing underneath.

And then messages of saying she was not feeling well and my dad saying if she needed to talk etc. Nothing weird there, just some friendship.

And then a message from him apologizing if the conversation had gotten to intense.

You get the point.
I’m 95% nothing more will ever happen outside those texts. I believe that maybe he just likes her as a friend and maybe this is some kind of thing that is just not big deal since they have been together for so long and are not getting separated.

Do I say anything? Do I just ignore it? Do I talk to my dad? Do I talk to my mom? Should I be worried?

TL;DR; : Found flirty messages from my dad(52M) to a massage therapist that treats us 3, me(22F) and my mom(50F). Nothing to explicit but alarming to the point that made me anxious. Don’t know if I should approach him or just ignore since my parents have been together for 25 years.