As said in the other mail, sorry to drop out of the conversation like that but as you probably guessed, IT connection vanished.
I understand this as an example of 'lie by omission'. So, first, I do not think that such a situation or similar is a lie in and of itself (if you do not tell). You can identify a lie easily as saying something you know is not correct, but 'lie by omission' is not identifiable the same way - so it seems to me. It depends entirely on the situation.
If I just saw the husband of a friend with another woman, I probably would think nothing of it and would not think of mentioning it, unless it happened to come up in a conversation.
If said friend had talked to me about being afraid that her husband was unfaithful, I guess I would mention it.
If said husband was kissing the woman passionately, I would have to consider whether it was none of my business, or if I had an obligation to tell.
Hm. I think the concept of lie by omission only makes real sense to me in situations where I have a clear obligation to inform of something, and do not.