1. A computer-implemented method comprising: detecting a current context associated with a mobile computing device, the current context indicating a physical location at which the mobile computing device is located; determining autonomously by the mobile computing device, based on the current context, whether to switch the mobile computing device from a current mode of operation to a second mode of operation during which the mobile computing device monitors ambient sounds for voice input that indicates a request to perform an operation; in response to determining to switch to the second mode of operation, activating one or more microphones and a speech analysis subsystem associated with the mobile computing device so that the mobile computing device receives a stream of audio data; identifying changes in the voice input in the stream of audio data, wherein the changes include at least one of the following: a pause in the voice input, a change in speed of delivery of the voice input, or a change in tone or inflection of the voice input; determining whether the voice input in the stream of audio data is directed at the mobile computing device based, at least in part, on the identified changes; determining whether the voice input in the stream of audio data indicates a request to perform an operation based, at least in part, on the determination of whether the voice input is directed at the mobile computing device; and providing an output on the mobile computing device that is responsive to the voice input that is detected in the stream of audio data and that indicates the request to perform an operation.