is it noticed if your spouse is having an affair or even, regularly visiting his "second home" ?

If there is no outright problem between the husband & wife, and no signs of abuse or violence - then you should not stick your nose into their lives.

You don't know where he goes, or what he does, but she probably knows. It may be just a job at a gym, or as a coach, etc. Maybe he's a security guard at night.

Mind your own business, and don't interfere.

It'll cause problems for your friendship if you get involved. Stay out of it.

I'm almost 100% sure that she has asked herself that very question and is either purposely avoiding it, or else they have some kind of pre-arrangement that is mutually agreed upon (like an open relationship).

Furthermore, it may have nothing to do with another woman. That could have been his behaviour over that one week, because they had guests, and he wanted to get away and have some peace.

Either way, it's not your place to meddle, unless she brings it up. IF she asks you what you think, feel free, otherwise you don't know how she'll react, and you may be the one that gets kicked to the curb.

Mind your own business. Some people like to work out late at night.

Stay out of other peoples business.

No, not unless you have photos. It is also a possibility that she doesn't care or they have some type of arrangement, so it would be wise to ignore their personal life.

The rest of you saw something after one visit....she lives with him ALL THE TIME. She may be purposely blind.

Honestly?...have him followed a few times, and then, if there is reason to tell, please do so. If not, you should not upset her over assumptions.