I've never heard of any colleagues doing it either. I would never snitch if it happened, but I would certainly consider that person untrustworthy or think that they have an undisclosed condition that is between them and HR.
As you covered in your post, we don't write code 8 hours a day. This is normal. Meetings, context switch and times when focus is unachievable are expected and mostly understood by my experienced management. So are times when we are monitoring technology advancement and keeping an eye on the industry and community (e.g. lurking at HN but not overly posting and commenting) and times of social activity between workers (e.g. talking about the weekend).
Most of us take mental health breaks between meetings and need to take time to think through issues. Many also need to periodically break focus to think about personal life issues or hobbies. As long as you generally strive to work, this is not a problem either.
But walking into a stand-up and then lying about your day? To me, that's unethical.
The real question to me is:
Do you do that on purpose or is this something that happens even if you are trying to focus? There are many conditions, from stress and anxiety to diagnosed mental health issues that can prevent you from focusing. Burn-outs and bore-outs also come to mind.
In my eyes, the fact that you asked the community about it shows that you have some form of concern towards that behavior. Would you say it is negatively impacting your life?