How do I know if someone copied files from my computer?

Determining if someone has copied files from your computer involves examining various digital footprints and system activities. While it can be challenging to find definitive proof of a copy operation alone (as it often doesn't leave a unique "copied" log), a combination of checks on file properties, system logs, network activity, and the use of monitoring tools can provide strong indications.

Key Indicators to Check for Suspicious File Activity

To ascertain whether files have been copied, you should look for anomalies across different aspects of your computer's operation.

1. Review File Access Timestamps

Files and folders on your computer store metadata, including timestamps that record when they were last accessed, modified, or created.

Last Accessed Date: This timestamp records the last time a file was opened or read. If a file that you haven't recently opened shows a recent "Date Accessed" that coincides with a period of suspected unauthorized activity, it could be an indicator.
- How to check: Right-click on a file or folder, select "Properties," and look under the "General" tab for "Date accessed."
- Important Note: Many modern operating systems (especially Windows since Vista) and certain applications may not reliably update the "Last Accessed" timestamp for performance reasons, or they might disable it by default. Therefore, this indicator might not always be conclusive on its own.

2. Monitor File Modification Dates

While copying a file typically preserves its original modification date on the copied version, an unauthorized user might have modified the original file on your system before or after copying it.

Last Modified Date: Check the "Date Modified" timestamp. If important files you haven't touched recently have a new modification date, it suggests they were altered, which could precede or follow an unauthorized copy.
- How to check: Similar to access dates, right-click a file/folder, go to "Properties," or view columns in File Explorer/Finder.

3. Review System Logs

Your operating system keeps detailed records of various events, including user logins, file access attempts, and system changes. These logs are a critical resource for forensic analysis.

Windows Event Viewer:
- Open Event Viewer (type eventvwr in the Run dialog or search bar).
- Navigate to Windows Logs > Security.
- Look for Event ID 4663 (An attempt was made to access an object) and Event ID 4656 (A handle to an object was requested) for file access. You'll need to have Object Access Auditing enabled in your local security policy (Group Policy Editor: gpedit.msc > Computer Configuration > Windows Settings > Security Settings > Local Policies > Audit Policy > Audit object access).
- Also, check Event ID 4624 (Successful Logon) and 4625 (Account Logon Failed) to identify unusual login times or failed attempts.
macOS and Linux Logs:
- macOS: Use the Console app or log stream --predicate 'process == "Finder"' in Terminal to check for file-related activities.
- Linux: Check /var/log/auth.log for login attempts, and explore auditd logs (/var/log/audit/audit.log) if system auditing is configured to monitor file access.

4. Analyze Network Activity

Suspicious network activity, especially large outbound data transfers, can indicate that files have been exfiltrated from your computer.

Network Monitoring Tools: Use tools like Wireshark, TCPView, or your router's traffic logs to identify unusual data usage, connections to unfamiliar IP addresses, or data being uploaded during times you weren't actively using the computer.
Firewall Logs: Your firewall might log outbound connections, which can reveal unauthorized attempts to send data out.
Cloud Sync Services: Check the sync logs of cloud storage services (e.g., Dropbox, Google Drive, OneDrive) if they are installed, as files might have been copied to a synced folder that then uploaded them.

5. Utilize File Auditing and Monitoring Tools

For more robust and proactive monitoring, dedicated software solutions or built-in auditing features can track file operations in real-time or log them comprehensively.

Comprehensive File Monitoring Software: Various third-party tools are designed to monitor file and folder activities, including creation, deletion, modification, access, and copying. These often provide detailed reports and real-time alerts. They can track which user account accessed which file, and when.
Operating System's Native Auditing Features:
- Windows Security Auditing: As mentioned under System Logs, enabling "Object Access Auditing" through Group Policy allows the system to record detailed events regarding file and folder access in the Security Event Log.
- Folder Permissions and Ownership: While not directly for "copying," review file and folder permissions regularly. Unauthorized changes to permissions could be a precursor to data exfiltration. Ensure ownership and access rights are as expected.

Summary Table of Detection Methods

Method	Description	Best For	Limitations
File Access Timestamps	Checking "Date Accessed" on files/folders.	Quick initial check for recent activity.	Often disabled or unreliable on modern OS for performance.
File Modification Dates	Checking "Date Modified" on files/folders.	Detecting changes or tampering with original files.	Copying alone doesn't change this on the source file.
System Logs	Reviewing Windows Event Viewer (Security Logs), macOS/Linux logs.	Identifying login times, file access attempts.	Requires auditing to be enabled; logs can be overwhelming.
Network Activity Analysis	Monitoring outbound data, suspicious connections using network tools.	Detecting data exfiltration (transfer off device).	Requires advanced tools and knowledge to interpret.
File Auditing Tools/Software	Using dedicated software or OS features to log file operations proactively.	Comprehensive, real-time monitoring and alerting.	May require prior setup; some tools can be costly or resource-intensive.

By combining these methods, you can build a clearer picture of whether unauthorized file copying has occurred and identify potential culprits.