You might not notice this one at first because it does not involve keeping secrets and hanging out with new friends. If it happens for a period of time, then you will notice it. You know that your partner took a shower in the morning before going to work but in those days when they say that they worked overtime as soon as they come back home, they jump in the shower again.
They do this for a pretty obvious reason and that is to remove the smell from the person they had an affair with. There are other ways to discover that. You can volunteer to do the laundry for the whole week and start going over their clothes and try to find a scent that you're not familiar with, most probably a perfume from the opposite sex.