I'm premenstrual, which is why I can't judge this clearly.
My partner of 14 years has recently started mentioning a colleague at work more often, e.g. talking about how they have bad weather where she lives, or that she broke her leg, or whatever. She's in another office so they don't work in the same place, but they have zoom meetings and I know that she has his mobile number as she's messaged him and sent him photos of stuff (e.g. her Halloween decorations). My partner is 49 and I'd say this woman is about 35 judging by photos.
Today he left his laptop open while he went out for the evening and I saw that he had a Teams chat going with her. It was nothing sexual, but I'd say it was flirtatious (you couldn't mistake it for a chat with a male colleague), but the worse thing was there was a LOT of it - up to 30 messages a day, going back about six months. And most of it was not work-related - it was about her kids, her town, her hobbies, politics, music, etc. She is married but her husband got a mention about once.
I'm trying to be rational about it. If he worked in the same office as her these conversations are probably the equivalent of a daily chat at the water cooler, which is not a crime, and certainly I used to chat to male colleagues when I worked in the office - I probably even flirted with some of them.
But messages onscreen seem more intimate somehow, and they're certainly more private than a watercooler chat. And the fact that she's messaging on Teams AND texting him seems a bit full on. I don't know how often she texts him.
So AIBU to feel uncomfortable about it? It doesn't help that our sex life has taken a nose dive in recent years because he has erectile issues, so I'm already feeling pretty rejected.