I think you have confused your self between the different account uses.
***''Find My''*** is leveraging a given users iCloud account (the original owner) while your account is a second user account. Think how multiple people can share a desktop using different system accounts. In truth these can be independent or the same User ID just doing different things.
So the other user never removed his association to the system so he can lock or erase the system at any time remotely, he also knows where his system is located geographically (where you are).
As far as your stuff depending on the ACL your data maybe exposed! Even within your iCould storage.