I asked a question about a month ago. After about a week, activity stopped. Now suddenly it is near the top of the list and the box says "active today". But I can find nothing dated today, and no obvious changes. I have noticed a similar phenomenon on other old questions suddenly reappearing at the top of the list with no detectable dated changes. Is there any way to search for and find recent changes? If not, I think this is a desperately needed new capability. Alternatively, Questions should not be made active for trivial or invisible changes. Or every answer and comment should have two dates, one for originally posted, and one for last modified.
Your question was unanswered, and such get poked randomly to give them another chance (see Community user profile). It may be prevented by accepting some answer.
As for seeing all changes, you can click on the "Edited..." link (if the question was ever edited) or put its ID in such an URL
https://physics.stackexchange.com/posts/<ID>/revisions to see recent changes. In case of the question I think you are asking about i it looks like this. However, as you can see, Community pokes are not visible there.
It is by design. The StackExchange engine bumps questions up to front page periodically, if your question does not have answers with sufficient number of up-votes, and if your question does not have an accepted answer, and (if I remember correctly) if your question does not have a net negative vote. This is so that old questions may get exposed to new blood and maybe your question will receive an answer.
There is a user called Community that undertakes a number of useful background tasks, like take possession of up- and down-votes from deleted users (for more detail see e.g. Who is the Community user?). In particular, the system bumps old unanswered questions to the top of the front page every so often to help them get answers.
When this happens, the link to the question will show Community as the last active user on that question:
If there is some other name, then the question has had some activity recently. To be taken to the last active post in that thread, click on the timestamp: