@magicwillow
Typically our exchanges involve a generic "how was your day?". The respones is usually " it was fine" . If something out of the ordinary or particularly interesting happened that would entail more discussion.
Having lunch with someone would not be something usually mentioned because we both have lunch with many different people for different reasons. And by the end of the day, luncheon meetings have all but faded from memory.
I don't think that I have ever seen any of his lunch receipts , and I doubt seriously that he has ever seen any of mine. We both work, and there would never be an issue of how much was spent or what was consumed
I don't know If he cheats. I have no reason to think so, but I don't ask questions such than he would have a reason to lie. That may mean that I am living in a fools paradise.
Sometimes though, I think people who feel that.their behavior is unfairly or too often scrutinized will provide whatever answer that will lead to no further discussion .
If the OP no longer trust him, it really doesn't matter what he was about. The only question for consideration should be does she want to be in a relationship with a partner that for whatever reason she does not trust.
Life is too short to spend any of it having to play Sherlock in your relationships.