@Véronique Carbonneau - Public/visitor access is enabled for your board - this is indicated by the public/globe icon in on the Share button:

You can read more about this in the Collaboration with Visitors help center article:

https://help.miro.com/hc/en-us/articles/360012524559-Collaboration-with-visitors#h_01FHZ57SZWG9ZXSSTGWB0Z0K14

Encountered the same problem today. A “visiting builder” entered my board and delete everything. I and my team members haven’t share the link to the public (only shared it with several invited guests), why this could happen? If I share the link to invite a guest into the board that can view and edit, it doesn’t mean other guys who do not have the link can enter the board as a guest right? Really curious about that.

Because you share the link with invited guests but when they went on Miro they didn’t create there profile.

You can always go back in the history of your board annd Go back to the changes. 

You can go in the sharings settings to make sure they need to iddentify themselves before going on your board. 

@JoeyQ - To clarify, Visitors and Guests are two distinct roles in Miro. For example, you said:

I and my team members haven’t share the link to the public (only shared it with several invited guests)

Visitors

Visitors are defined as those who access the board once you have set the following to anything other than “No access”, i.e., Can comment, Can View, Can edit:

Once this setting is enabled, anyone with the link will have access to the board, regardless if they have a Miro account or not (which is also referred to as unregistered or anonymous access). Note: whether they can actually see the board content or not will depend on if you set a password for the board.

 So, from what you said, you have enabled the above setting for Visitors to access the board. Unregistered/non-signed users will be assigned a fun, random name:

Inviting Visitors to add their names

Registered users who sign in appear on the board with their real names while Visitors with edit access are automatically assigned a name by us (e.g. Visiting Inventor). 

For a more collaborative way to engage with Visitors, some board owners and co-owners can allow non-registered users to add their names when they join the board.

Guests

The Guests feature is available on all plans, except for the Free Plan. Note: Only the Business and Enterprise Plans support Guest editors, while the other paid/Education Plans only support Guest Viewer or Commenter. 

Once you have added a Guest to a board, you team space will show up in their Miro dashboard and will only list any boards that you have added them to as a Guest - Note: They cannot create boards in your team space, or see any Projects.

More on the Guests feature here → https://help.miro.com/hc/en-us/articles/360021415119-Collaboration-with-Guests

And you can compare Visitors, guests and members here → https://help.miro.com/hc/en-us/articles/7045408248594-Visitors-guests-and-members