How can I tell why I have access to a file share on Windows Server

I have a file share on a Windows 2008 R2 server in a AD domain (call it \SECURESERVER\STUFF) and I am not sure if I have the share and folder permissions set up right. I noticed the problem when I set up new server (WORKGROUP\FOREIGNSERVER) that was not joined to the domain and tried to copy some files off of \SECURESERVER\STUFF. I was surprised to find that when I tried to access the files, it did not prompt me for a username and password and proceeded to give me full access to the files. That worried me so I tried the same thing on some workstations that were not in the domain and they did NOT have the same behavior (they did prompt for a username/password as desired/expected). So, I think there is something peculiar about FOREIGNSERVER. I am logging into it with a local admin account, but my domain and SECURESERVER should know nothing of this server. I've carefully gone through the share and folder permissions on the share but I can't find the reason that FOREIGNSERVER has access. How can I find out why FOREIGNSERVER has access to SECURESERVER?

Were you using an account that had the same account name and password on both servers?

If you are logged into secureserver with domain account: joe and password: 123 and have a local account joe and with password 123 on the workgroup server, permissions will generally flow and it will login to the workgroup server with the local account.