A detector in a mobile device receives input from a modem, determines whether the mobile device is indoor or outdoor based on the modem-supplied input, and stores in memory a binary value to indicate an indoor-outdoor state of the mobile device. In certain embodiments, the mobile device includes a modem of a cell phone and the modem-supplied input includes an estimate of a power delay profile. In some embodiments, the detector extracts a feature from the modem-supplied input, and uses the extracted feature with a classifier, to output a state and a probability of the state. In these embodiments, logic in the detector compares an empirically-determined threshold, against a probability output by the classifier, and when the threshold is exceeded, the state determined by the classifier is output as the state of the mobile device. In other embodiments, the classifier outputs the state of the mobile device directly (without probability).