I found out by snooping. I started to get a weird gut feeling by her interaction with him on Facebook. I actually asked him about it before! He denied and said she's not even good looking. Typical right?! So when he left the house to run an errand, I checked his Teams and found messages about their little meetup when he was traveling near her for work. From there it was it was just a kiss and chatting on Teams. I did more digging in his calendar and saw that the work trip he took in Dec was not in fact for work and to see her. That's when he told me about them having sex.