A passive detector (10) includes a receiver (11) configured to collect passive radiation (12) in an environment, where detailed information about a portion of the passive radiation is estimated as a baseline of the passive energy. The passive energy is generated by a passive source unrelated to the detector. A monitor (24) is configured to measure a fluctuation in the baseline. A decision module (34) is coupled to the monitor to determine whether the fluctuation represents a presence or motion in the environment. Detection methods are also disclosed.