You will have to be carefull if you think that a private Team should be private, meaning that no-one should see it. A user needs to be invited or approved to see the content of a private Team. A new change to Office 365 will make the private teams discoverable via search and Suggested Teams. If you have private teams with sensitive information in Nams or Description you might want to tell your users to rename them. See below.

Note : It might be already like this on your tenant…

Major update: General Availability rollout started

Applied to: All customers


Starting March 23, 2018, private teams will be searchable in Microsoft Teams. This feature update will make it easier for people to share and join Teams.

[How does this affect me?]
Until now, it was not possible for users to find and request access to a private team in the Teams app. Users had to either be invited to a team or search for the underlying group in Outlook in the Outlook app.

Now, private teams will be discoverable via search and the Suggested teams gallery, making it easier to share and discover private teams across an organization. If a user requests to join a private team, the team owner will receive a notification and can approve or deny the request directly in Teams.

[What do I need to do to prepare for this change?]
We recommend that you inform team owners about this change. If team names or descriptions include sensitive information, team owners may want to update them before this change occurs on March 23, 2018.

If you wish to hide a private team or group, Office 365 admins can use the Set-UnifiedGroup PowerShell cmdlet to hide specific groups or teams
(in Outlook, Outlook Web App, and Microsoft Teams). Specifically, use the HiddenFromAddressListsEnabled parameter, which you can apply to individual groups. For further instructions, please click Additional Information below.