DO NOT BRING THIS UP TO HIM UNITL YOU HAVE THE AFFAIR CONFIRMED.
THEN.. and only then do you approach him.
You don't approach him by asking. You approach him by TELLING him you KNOW. It isn't up for negotiation or his side of the story.
When you have it confirmed it, it makes your approaching him have validity and gives you more strength than if you just go ask him.
You never go in asking. You are only opening yourself up to be lied to. I, for one already know he is having one or just ended one. Of course I am not the one that needs to be convincedd of it. YOU ARE.
So far you have been doing some minor investigation. This was good. Now you need to keep on the trail and find out who it was really with etc. etc... You need more evidence.
Your husband is a cop. Do you think that when he has evidence against a suspect that he stop investigating if the suspect denies that he is guilty? Of course not.
Put your effor into finding out the facts here. You are trying to go in and get the truth from the suspect. Just because a suspect denies involvement in a case doesn't prevent the cops from pursuing more evidence by other means. They don't just say, "ok, said suspect says the are innocent, so the case is solved"...
Channel your energy into finding out more. You can confront AFTER you have your ducks in a row. If you don't then you are letting the suspect run your case for you. Not good. There is plenty of evidence that he has been up to something. Men just don't up and move into their sister's house, and then come back home. That in itself is suspect.