[SOVLED] A user suddently became the Owner of every file and folder?

[SOLVED]

Someone gave that team member Site Collection rights, that's why, it's now been removed, access rights is back to normal.

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A very bizzare question, one of our team users suddenly became a OWNER of every file and folder!!?

I'm the admin/owner and I can see his Members and Permission setting under the user.aspx is still under the member's group setting, not an Owner.

How could this have happened and how do I remove him from having access to all files and folders including those confidentail ones?