Originally Posted by
biminiblue 
He may have just been on the crazy train.
I agree with bimini.
He may well have been on the crazy train (from your descriptions he was the engineer).
Your instincts may be spot on but the facts, you don't know. He may just have been thinking about cheating or been discussing it with someone who was - you just don't know.
You have to decide what this knowledge is worth to you. You can ask him and decide if he is telling you the truth or hire a detective.
You get to decide. It's easy for us to say just brush it away but that might not be your reality. I would really talk this over with your therapist/counsellor there are no doubt tools that can help you deal with these types of thoughts.