New folder "ThumbnailFolder" in Documents

A new "Thumbnail" folder appearing inside your Documents folder on macOS Taho26.6.2 would have been created by a third-party application, cloud-sync client, or a media-management tool rather than being a native core system directory introduced by Apple.

Potential Causes

• Third-Party Apps: Software like design tools, video editors, or photo organizers often default to creating local cache or preview folders in user directories like Documents when handling local file previews.
• Cloud or Sync Services: Certain cloud storage integrations or external drive companion apps (such as specific configurations for Dropbox or localized syncing tools) may generate a local cache/thumbnail directory if they misidentify or re-map default paths.

What You Should Do

• Check Contents: Open the folder to inspect the files inside. If the files match a specific app you use (like an e-book reader, photo suite, or design utility), that application is the culprit.
• Relocate or Test: Try moving or renaming the folder. If an app complains or recreates it, you can check that app's settings to change its cache or temporary file storage location.

Also, note that the current version of macOS Tahoe is 26.7.1.

See: Update macOS on Mac - Apple Support

Maybe a leftover from some Android device or Windows?

Thanks for detailed explanation and followed your advice. Probably related to a crash of a pdf reader before I noticed the folder. Deleted the folder. Consider it solved.