In signal-detection research, occasionally a subject will report detecting a signal even in the signal's absence. What is this called?

False alarm

Explanation:

In an experiment in the field of signal detection it is the task of the observer to discern the existence of a stimulus. In other words to detect a stimulus in the midst of background noise.

When there is signal (stimulus) present but the person does not detect it is known as a miss.

When there is no signal (stimulus) and the person says there is one it is known as false alarm.