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I am thinking of checking if someone replied to a thread which you have posted. I guess, it will come in two parts.

1. Recording users in thread when someone replied

2. Send dismissable notifications when those users visit next time.

correct ?

In some place in particular or in "any"? A kind of alert?
I feel you could have a look into Post_Controller, at a certain point (sorry, I don't have the code handy at the moment) the code does already something like that in order to send the notification (new replies in particular), so you could "attach" to that functionality.
Then, depending on what exactly you want to achieve there may be better ways to handle the thing.
One off the top of my head is the just have a table | user | id_topic | new_replies | and toggle "new_replies" to 1 when someone posts a reply (with the method described above), and turning it to 0 when the member opens the topic in the browser.