'Impossible travel' refers to a situation where a user account is accessed from geographically distant locations within a timeframe that is too short for normal travel to occur, suggesting the use of compromised credentials. This is an important red flag for security analysts as it may indicate an account takeover by an unauthorized user. In contrast, 'Concurrent session usage' may raise concern but does not imply physical impossibility, 'Account lockout' is a response to suspicious activities and itself is not an indicator, and 'Time-of-check (TOC)' relates to a specific vulnerability type concerning the timing of security checks.