5. We notice and understand a body language
Women are known to have a sixth sense, especially if it concerns relationships. We always notice and analyze all the small details in another person, and that includes a body language of our partners. So we usually are aware of his sympathy to a colleague, which we have never met.
Once we notice something suspicious, it starts bothering us, though we may not be aware of what exactly annoys us. And if a woman suspects something, she will be unbearably upset when her man does not spend time with her but stays in the office.
If you are going to cheat on her, do not hesitate she will figure it out. Your body language will send her clues, no matter how hard you will try to hide it.