I found out my ex of 12 years was cheating in a similar matter. For some reason she liked taking screenshot of conversations, I had set up Amazon pictures auto backup on her phone at her request cause she was afraid of losing 16 years of pictures. One day I was looking through the backups cause my phone was also set up and I was looking for an old picture I no longer had on my phone. I ended up finding plenty of screenshot of her texts with an old school boyfriend she had been cheating on me with for almost 2 years. Nothing physical as far as I could tell but I can't say for certain it didn't happen, emotional cheating is just as bad for me anyways.
I also saw that some screenshot were from Instagram and I knew her tablet was logged in so I checked and it was all there. Worst part was, that she would often be texting him when we were together doing things and basically telling him she wish she was there. Worst 3 months of my life while I got my ducks in a row so I could leave without issue.

I found out she met him at least twice on her yearly trips back to her home country.