According to embodiments, techniques for detecting probe-off events are disclosed. A sensor or probe may be used to obtain a plethysmograph or photoplethysmograph (PPG) signal from a subject. A wavelet transform of the signal may be performed and a scalogram may be generated based at least in part on the wavelet transform. One or more characteristics of the scalogram may be determined. The determined characteristics may include, for example, an energy decrease, a broadscale high-energy cone, a regular, repeated high-scale pattern, a low-scale information pattern; and a pulse band. The absence or presence of these and other characteristics, along with information about the characteristics, may be analyzed to detect a probe-off event. A confidence indicator may be calculated in connection with probe-off event detections and alarms may be generated when probe-off events occur.