What should I do? Saw some flirty messages on bf phone

I'm 26F. My bf is 32M. Been dating for two years plus.

We were having some friends over and I was selecting music. And this girl named Angela texted him saying something about picture of a cat. I was like wtf are they sexting?? So I clicked and no it's really a cat picture. But I read the rest of it quickly. And he was really sweet in a way of caring whether she ate, she’s done with work. Like good morning good night texts and updates throughout the day. Like when he was cooking for me today and last night, he texted her too. There is some call records not lasting too long today and last night. Multiple times even when I was home and working or while I was asleep. I looked at the media gallery really quickly and it’s all memes or pictures of food and pictures of her cat (animal).

Is there a good reason for this? I’m not naive, just asking in case he comes up with a good excuse and would gaslight me. I’m trying to steel myself.

We have a two week trip to Japan in 3 days. Should I talk to him before or after? I’m non confrontational and I don’t want to break up (if it arrives there) before the trip. I’m planning to talk to him after the trip but one of my best friends say I won’t enjoy the trip anyway if I don’t talk about it now. And I know it’s best to communicate transparently in a relationship so I’m torn.

Please advise! He’s been a great bf so far in these two years, not a single red flag. He always updates me everyday about his daily life, communicates well, cooks for me, is physical and verbally affectionate and I always trusted him. I don’t know his passwords so I cant check his accounts. The girl is from our home country. So he is not meeting her physically.

TL DR saw some flirty messages on bf phone. It's not explicitly cheating (?). What should I do?