The growth in popularity of hand-held mobile devices has fuelled
research exploring how to harness the collective abilities of sensors
attached to these devices.  One area of development has been urban
sensing, which explores building a crowd-sourced wireless sensor
network using consumer mobile devices.  Urban sensing participants use
their devices to capture information about their surroundings to
contribute to an urban sensing system.  Existing research has explored
protecting the privacy of the urban sensing participants, through
anonymization and aggregation of collected data.  We are interested in
the privacy of bystanders who may be inadvertently affected by nearby
urban sensing data collection.  There are difficult aspects to this
problem, as we must weigh the privacy of bystanders against the
privacy of urban sensing participants.  We describe NotiSense, a
simple system that provides useful notications of nearby sensing
activities to those who choose to subscribe.  We evaluate a prototype
implementation of NotiSense and its use of Wi-Fi to provide
notications.  NotiSense is a good approach to enhancing the privacy of
bystanders and opens up interesting challenges for future work.