I guess, if found my solution myself with “content admin” https://help.miro.com/hc/en-us/articles/360012777280-Content-Admin-permissions-for-Company-Admins#h_d8152ee0-3819-40d0-ac4a-3f53b32fcaca

So it is not possible the way I intended,  but I can search for that board at least.

HI ​@Timo, thanks for reaching out!

There isn’t an “admin key” that gives a full view of all spaces and boards in your team. However, as a Company Admin, you can assign yourself the Content Admin role. This allows you to search for and manage all boards and projects across the account, even if the original owner has left.

Here’s the help article you linked for reference: Content Admin permissions for Company Admins.

Thanks again for sharing back your findings — it’ll be helpful for others with the same question!