This is weird. I have an extension that is not available through the webstore because it's marked as private and is still pending review. However, I see the weekly users count increasing.
When the counter was at 1, I thought it was somehow tracking my own usage of the locally built .crx extension because the manifest contains the public key. But now it's at 10?
Is there some kind of accidental public link to pending extensions that I don't know about?
When you published your item as private that means only the trusted testers you set could see and install your extension. Besides that, the team in Google which handles the manual review and marked your item as Pending Review (maybe due some policy violation) could see and install your extension ether (probably for testing). Those status data would be counted as well probably. To my best knowledge, there's no kind of accidental public link to pending extensions like you said.