A mobile device comprising: a processor configured to execute a two-way messaging application, comprising: a message leaving component configured to leave a message to be retrieved via a luminaire by: generating message content according to message generation inputs from a user of a mobile device; using at least one light sensing device to extract, from light emitted by the luminaire, a code embedded in the light as non-visible modulations; and using the received code to cause the message content to be stored in association with the luminaire for retrieval by one or more recipient users; and a message retrieving component configured to retrieve a message via a luminaire of the lighting system by: using the at least one light sensing device to extract the message content or data for retrieving the message embedded in the light as non-visible modulations, and output the message content via a user interface.