Hidden Site Collections in SharePoint Online

Recently I was given a task to find site collections that were not visible in admin center but actually users were using them, and the it manager didn't have access on them.

I found two scenarios where this situation is possible. So here is my analysis on them:

• Scenario I - ContentTypeHub

SharePoint online has a hidden site collection named "contenttypehub" (URL - /teams/contentTypeHub). You can host your content types here. They will be available in all the site collections. 

• Scenario II- SharePoint Group Sites

If you have created Office365 groups, you will have a separate site collection of each group created where group members can share & collaborate. 

Please refer below for more details on SharePoint groups:

• https://support.office.com/en-us/article/learn-about-office-365-groups-b565caa1-5c40-40ef-9915-60fdb2d97fa2?ui=en-US&rs=en-US&ad=US
• https://www.avepoint.com/blog/office-365/understanding-office-365-groups-technical-limits-and-boundaries/