Why non members of my private group can see my posts ?

Bonjour,
I've recently created a private group and for the setting I chose "visible" (anyone can see the group) and "closed) ( = Only members will be able to see who's in the group and what they post.)
So I thought that non members couldn't see my posts;
However, a non member just told me she could see my posts.
I don't understand. Could you help me Thank you by advance