To think this is flirty.

[GRJ212]

I know I do have major trust issues following being cheated on repeatedly in a previous relationship. So I do struggle sometimes with knowing what is appropriate and what isn’t. Been with DP for ten years and no cheating but I do still struggle with trust at times.
He’s been in a new job for a little under a year and yesterday I saw his teams chat between him and a female colleague open on his laptop at our kitchen table. I’ve had a weird gut feeling about this lady for months. He rarely mentions her name to me despite me seeing it pop up on teams all the time. I had a look through the chat and there were a fair few things that really didn’t sit right with me but now doubting my own judgement.
At one point he mentioned her birthday and she asked if he would be wearing a particular item of clothing, to which he responded “if it would make you happy”. They also have a sort of in joke between them, in which they call each other made up characters and he’d made her a little picture of their two characters together. At one stage she had mentioned him throwing her under the bus to which he had responded “I’d never do that to you. Ever.” I don’t know it just feels a bit much. He also seems to regularly check in and ask if she’s ok etc.
I just cannot work out if I’m over thinking it. If this was the way he was with ALL colleagues I think I’d feel a bit better about things. But it’s not. Just with this woman. And now I can’t get it out of my head.

I wouldn't be happy with that. He's getting close to the line but not over it. A bit of joking about is fine but he needs to be more professional and less flirty.

I'd tell him you've seen it and you're not impressed so he needs to knock it on the head. How he responds will tell you if you have a problem. A decent DH will be mortified that he's upset you and will catch himself on.

Well their relationship is certainly not just professional.
Having " in jokes" between the two of them and talking about wearing certain clothes specially for each other sounds like more a romantic relationship developing.
I think you need to have a talk with your DP about boundaries.

Sorry I should have said, I did confront him on it. He denied it being flirty and said there was no intent behind it. He said he would be more aware in future chats with her. So essentially he said all the right things. But isn’t that just what anyone would say when confronted? And realistically how would I ever know. I don’t know I am up and down now if I can trust him I suppose. But it seems a hell of a minor thing to throw away ten years on. Having said that, I cannot be in a relationship without trust. Been there, done that.