Your husband does not sound like a philanderer, but the fact that he works away from home has given him the opportunity to stray and often that is all it takes.
Further, while competition and betrayal are the main thorns to female friendship; men are plagued in almost equal amounts by two additional issues, lack of frankness and a fear of appearing unmanly.
The work he is certain of North Korean soldiers protecting the tunnel. This North Korean defector was a senior intelligence official within the military.