What indicates a lavatory smoke detector has been activated?

The correct answer is the smoke alarm sound. When a lavatory smoke detector is activated, it produces a sound that alerts crew members and passengers to the presence of smoke. This auditory signal is crucial as it serves to notify everyone in the vicinity of a potential fire hazard, facilitating prompt responses to ensure safety.

Other options, such as power failure or circuit breaker issues, may indicate problems with electrical systems but do not specifically signify that the smoke detector has detected smoke. Hot spots might indicate areas of excessive heat but do not directly correlate with smoke detection either. Thus, the sound of the smoke alarm is the definitive signal for activation, making it the correct choice in this context.