You are excluding many valid reasons for this to happen.
His colleague would easily notice someone viewed an email in his in-box.
You can set an email to Unread in pretty much every mail app I've ever used. So how would the colleague see that the mail was read, but then put back into an unread state?
OR his colleague could easily go back to the original email and download the original attachment
He could, yes. But why? He has no reason to suspect that anything has changed in the list since he has no knowledge of anyone having been on his computer.
If you copy a file to a USB stick, do you then always re-check the copied file bit by bit to make sure that everything copied correctly? No, you don't. No one does. Unless there is a problem with the copied file, in which case you would try to copy it again.
If the colleague suddenly noticed that there were 99 people on a list that says there should be 100 people, he might be willing to recheck his sources. However, if there is nothing that leads him to believe there is anything wrong with the list; he has no reason to recheck his source.
Given his colleagues are all professional, how could this happen?
You can't expect a fictional character to know everything the viewer knows. More often than not, them not knowing it is the whole point of it all.
Not every person is aware of every piece of evidence that exists everywhere in the world. And even if they were, they would not continually recheck everything; because this all takes up valuable time, and there is no reason to suspect that there is anything incriminating to be found in the first place.
For a similar reason, not every security tape ever recorded will be watched by someone in the future. They will only be watched if anything interesting happened during that time.
If nothing interesting happens, why would you look back? What would prompt you to do so?
To prove my point, I have written some incriminating evidence about me in this answer. However, I have edited it out again.
Be honest: before I told you that I had put something incriminating in my post, did you already check my edit history?
You didn't. Because you had no reason to suspect that there was any evidence to be found.
And that's the whole point. Unless someone (or something) makes the Asian colleague think that there is anything wrong with the list of registered people, he has no reason to check it again.