Can a windows domain admin access non shared files on a PC connected, but not joined to the domain?

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http://serverfault.com – My company allows users to bring their own devices into the office and use the internal network due to our fast internet connection. These devices/PC's are not joined to the company Windows Domain. Can anyone on the domain, including domain admin accounts, access files or shares on these devices/PC's without the owners knowledge? Are users personal files safe? (HowTos)