  A major challenge of any search for gravitational waves is to distinguish
true astrophysical signals from those of terrestrial origin. Gravitational-wave
experiments therefore make use of multiple detectors, considering only those
signals which appear in coincidence in two or more instruments. It is unclear,
however, how to interpret loud gravitational-wave candidates observed when only
one detector is operational. In this paper, we demonstrate that the observed
rate of binary black hole mergers can be leveraged in order to make confident
detections of gravitational-wave signals with one detector alone. We quantify
detection confidences in terms of the probability $P(S)$ that a signal
candidate is of astrophysical origin. We find that, at current levels of
instrumental sensitivity, loud signal candidates observed with a single
Advanced LIGO detector can be assigned $P(S)\gtrsim0.4$. In the future,
Advanced LIGO may be able to observe single-detector events with confidences
exceeding $P(S)\sim90\%$.
