[donmcronald]

In my case though, the Google Search Console explicitly listed the exact URL for a newly created shared folder as the cause.

https://photos.example.com/albums/xxxxxxxx-xxxx-xxxx-xxxx-xx...

That's not going to be gleaned from a CT log or guessed randomly. The URL was only transmitted once to one person via e-mail. The sending was done via MXRoute and the recipient was using GMail (legacy Workspace).

The only possible way for Google to have gotten that URL to start the process would have been by scanning the recipient's e-mail.

[mkl]

Not quite. Presumably the recipient clicked the link, at which point their browser knows it and, depending on browser and settings, may submit it to Google to check if it's "safe": https://support.google.com/chrome/answer/9890866#zippy=%2Cen...

[donmcronald]

Good point. Thank you.

I've read almost everything linked in this post and on Reddit and, with what you pointed out considered, I'd say the most likely thing that got my domain flagged is having a redirect to a default styled login page.

The thing that really frustrates me if that's the case is that it has a large impact on non-customized self-hosted services and Google makes no effort to avoid the false positives. Something as simple as guidance for self-hosted apps to have a custom login screen to differentiate from each other would make a huge difference.

Of course, it's beneficial to Google if they can make self-hosting as difficult as possible, so there's no incentive to fix things like this.