The best explanation for this anomaly is that the terminated employee's credentials have likely been compromised. Since the login attempts are occurring from a foreign country shortly after the employee's departure, it's improbable that they would be traveling there and trying to access the company's systems. The other options do not align as closely with the scenario provided, as social engineering and hardware failure would not explain login attempts from a foreign location, and a distributed denial-of-service attack does not relate directly to unauthorized access to a user account.