Something feels off about my fiance's feelings towards his ex.

I saw him checking his kids' cell phone and I asked what he was looking at. He said he was looking to see if their Mom called to talk to them while he and I were out. I asked him why it mattered and he said, "I was eating out at lunch today with a coworker. She came in with (her bf) saw me and they turned around and went back out. So I just wanted to see if she called the kids today." So I asked him how them bumping into each other had anything to do with whether or not she would call and he said "I just wanted to know". I had never seen him check before so I said, "Well what would you have done if we had gone into a place she was eating at, wouldn't you leave so you wouldn't have to eat next to her or look at her?" "Hell no, I'd stay. Shove it in her face that I had just as much right to be there as she did"
I said humm,... and walked away ending the conversation.
It bothers me but I don't know why,
I don't think he still loves her. They fought constantly when they were married, (his kids have told me they remember and although were upset at first about the divorce they say it is quieter now,lol)
But I don't know what about it rubs me the wrong way.