As per the Scrum Guide content on daily standups, the three questions for discussion are:
- What did I do yesterday that helped the Development Team meet the Sprint Goal?
- What will I do today to help the Development Team meet the Sprint Goal?
- Do I see any impediment that prevents me or the Development Team from meeting the Sprint Goal?
All of the questions focus around the Sprint Goal, not the tasks that are on the board. Again, per the Scrum Guide, the Sprint Goal is created in Sprint Planning and it defines "an objective that will be met within the Sprint through the implementation of the Product Backlog, and it provides guidance to the Development Team on why it is building the Increment".
Everything that your development team does should, ideally, be helping the team progress toward the Sprint Goal. These may be unplanned activities that aren't on the board that had to be done, or they could be things at a lower level that may have been considered and estimated, but at a lower level than an item on the board.
I would say let your team talk about everything that they did yesterday. If they are talking about things that don't help the team reach the Sprint Goal, then someone should bring this up, especially if there were other things that they could have done that did move the team closer to completing the Sprint Goal.
One exception may be if an individual is supporting multiple Scrum teams. In the meeting, they probably shouldn't talk about everything they did yesterday, but what they did in support of the team that is currently having the standup.
The Sprint Retrospective is a great time to talk about this issue with the team. There are plenty of questions to consider:
- Is the team underworked on items related to the Sprint Goal?
- Is there too much unplanned work?
- Where is the unplanned work coming from and who is authorizing it?
- Why are people working on things not on the board?
- Should we be showing more details on the board to tie the things you do to items on the board more easily?