Mystery User Accounts

At least they were mysterious to me. I have a disk that I do all downloads too. On that disk I found some (four) files that were named like s-1-etc. etc. A Long file name. I checked the properties and decided they were not needed. Deleted same. Later I was checking some files on the same disk and discovered in the security panel I had two "unknown user account" which had matching numbers to the files I had deleted.
I started using the msc files that handle accounts and these unknown accounts didn't show up any where. The disk I previously mentioned was the only disk that showed these accounts in the list of user accounts. Other disks did not show them. I did a malware/virus scan with MS security essentials and it came up with nothing. I removed the two accounts from the list and now everything appears ok. Is there an explanation for this?