how to get the im_marked event on slack API

I am developing a bot with the slack RTM API. I want to catch im_marked events.

My problem is that the RTM API does not receive any event when a user checks his private channel with the bot, with unread messages. I already tried with two different libraries.

I guess I am not looking at the right stage or I do not have the right use case. In what context do the RTM receive the im_marked event ?

You won't see im_marked for other users. From the im_marked documentation:

The im_marked event is sent to all open connections for a user when that user moves the read cursor in a direct message channel by calling the im.mark API method.

I believe the main use case for this event is to build a Slack client. There, you may need to know if the user read a message via another client.