I found a mystery item in Full Disk Access, which I don't recall authorizing but which was checked off as authorized. It was a solid black box with green lettering in the top left corner exec and called smbd. Any ideas why that would be there?

What's smbd? It asked the full disc access - Apple Community

smbd

You apparently needed it at some point.

It is activated when you turn on SMB file sharing in System Preferences > Sharing > File Sharing > Options

I looked around too and, since everything referenced file sharing and printing services to Windows clients, I figured "Well, that ain't me". So, I started this thread.

What's SMB? I checked my System Preferences > Sharing and all I have checked are File Sharing and Bluetooth Sharing. I don't see SMB.

Also, "It is activated when you turn on SMB file sharing..." Does that mean automatically? I always thought that Full Disk Access could only be granted if I unlocked the window and provided authorization.

SMB is found in System Preferences > Sharing > Options. Options appears up when File Sharing is highlighted. It refers to a file sharing technique used by Windows and many Windows based network resources as opposed to AFP (Apple Filesharing Protocol).