Has anyone seen an unwanted viewer suddenly appear on their PC?

Hello @MarkSTV333333_,

This is not intended to happen. You need to make sure to protect your devices when using TeamViewer.

This is why we created a guide on how to guarantee a safe usage of the software. Please check our Four Golden Security Settings here:

In your case, I recommend setting up an Allowlist on your device to prevent unknown users to connect to your device.