Key backups with an alt

Hi, a quickie this time.
If I give a set of keys to an alt for safe keeping and backup and he has them on a keyring while sleeping somewhere away from home. Can someone just come along and take them from his leanto?
I read on another thread it is a good idea to give an alt a set of master keys in case of an emergency or disaster. But if they are master keys, they can be copied. The thief could copy it and attune the new one to himself.
If I give a set of keys to an alt for safe keeping and backup and he has them on a keyring while sleeping somewhere away from home. Can someone just come along and take them from his leanto?
I read on another thread it is a good idea to give an alt a set of master keys in case of an emergency or disaster. But if they are master keys, they can be copied. The thief could copy it and attune the new one to himself.