Could OneDrive have resumed syncing, or was my PC compromised?

This sounds more like OneDrive was opened or re-enabled and started syncing its default folders again. A Windows update or a sign-in to a cloud account can sometimes restore or change OneDrive behavior. Check the OneDrive icon and settings, including which folders are being synced and whether Desktop backup is enabled. Also review your account’s recent sign-ins and connected devices.

If you need absolute certainty that the operating system is clean, back up only personal documents after checking them, then reinstall Windows from trusted installation media. That’s usually unnecessary based on what you described, though—it would be a last resort rather than the first troubleshooting step.

A clean full scan and the fact that the files came from a cloud service make an accidental sync configuration much more likely than malware. Before doing anything drastic, inspect OneDrive’s activity and your cloud account security history, sign out of unfamiliar devices, and enable two-factor authentication. Change passwords immediately only if you find suspicious sign-ins or reused credentials.

Simply joining an open Wi‑Fi network usually doesn’t let someone take over a properly secured Windows computer. The more common risk is traffic interception, especially on unencrypted sites, rather than someone remotely controlling the machine. Keep Windows and applications patched, leave the firewall enabled, and avoid entering sensitive information on that network.