I found 3 SharePoint sites that seemed not to be used in my tenant.
All of them had a name and URL that ended with _CVOwners_ and then a 36 character GUID
Where had they come from ? Why weren't they being used ?
I searched online (couldn't find anything) and then after a bit of digging realised that while we don't use Dynamics 365 Customer Voice, some staff had used it in the past and there I found the source of my _CVOwners_ sites.
It seems (someone will know better than I) that when a Customer Voice form is created, it also creates a M365 Security group AND a SharePoint site AND an Outlook account (and in some cases a Calendar too).
This post is just for those out there who have enough access to see the sites, but not enough to get into Dynamcis 365 Customer Voice and confirm where the sites came from.