@Zatiranoio - "Private" boards refers to access for other members of your Miro plan team and not the public. Because the Free Plan does not offer "private" boards, all members of your Free Plan team will always be able to see your boards. 

"Public" boards would be where you have set the Visitors setting, i.e., "Anyone with the board access" to anything other than "No access" and then then shared the link - anyone with the link could view your board. 

In summary, no, people will not be able to find your boards.

More on private vs. public boards here:

https://help.miro.com/hc/en-us/articles/360011114519-Public-boards-vs-private-boards

To double down on what Robert said, all boards (including those located in Free plan teams) are not searchable on search engines (such as Google, etc.), so no one can just "stumble upon" them.

The way they can get access to one of your boards is either:

• by being invited to that board or to the team where it is located; 
• if the board is shared via the public link, they can get this link from you or someone else with whom you shared the board at some point.