The Original-LL-Viewer has a confusing bug (probably since some Versions) and this Bug probably affected TPVs, that are close to the LL-Sourcecode:

In the moment, you open the Filedialog for uploading meshes, an Ghost-Avatar
appear in the South/West-Corner of the Region. Below the usual Landmass.
Every time you open this dialog, a new Ghost appear.

This Avatar is not detectable: Neither the Serverconsole nor Script-sensorfunctions and nor the big-map within the Viewer notice him. Only the nearby-list and the Mini-Map show the Ghost. You can zoom with the Camera to him. This Avatar is grey ruthed. And his UUID seems to be randomly.

This happens in SecondLife too. So, this avatar is no "intruder": It's an Viewer-internal phenomena.

Restart of the Viewer or temporarily teleport to another region will let vanishing the Ghost.

As i heared just now, this is a bad idea of LindenLabs. Older Viewer has hidden this Ghost-Avatar. Not to hide this "technic" is a bad idea, because some land-owners are probably confused.

Here it is explained:

https://jira.secondlife.com/browse/VWR-28841?

He's the guy that carries in your assets for the preview window, among other things. He's always been in and visible, just his minimap dot wasn't displayed until v2/v3 viewers.