An apparatus for detecting an object in an environment, the apparatus comprising: a processor configured to: control an active reflected wave detector to measure wave reflections from the environment to accrue measured wave reflection data; process the measured wave reflection data to detect an object in said environment; in response to detection of said object, determine whether a first predetermined condition in respect of the object is met; and if the first predetermined condition is met, control an output device (216,218) to output a deterrent; wherein the processor is further configured to: process further measured wave reflection data accrued by the active reflected wave detector to determine whether, after a predetermined time period has elapsed after output of the deterrent, a second predetermined condition in respect of the object is met, wherein the second predetermined condition comprises that the deterrent was output based on the object being located in a predetermined area within a field of view of the active reflected wave detector, and the object remains in said predetermined area after a predetermined time period has elapsed after output of said deterrent; and if the second predetermined condition is met, control the output device to output a second deterrent, wherein the second predetermined condition is based at least on a location of said object in the environment. control an active reflected wave detector to measure wave reflections from the environment to accrue measured wave reflection data; process the measured wave reflection data to detect an object in said environment; in response to detection of said object, determine whether a first predetermined condition in respect of the object is met; and if the first predetermined condition is met, control an output device (216,218) to output a deterrent; wherein the processor is further configured to: process further measured wave reflection data accrued by the active reflected wave detector to determine whether, after a predetermined time period has elapsed after output of the deterrent, a second predetermined condition in respect of the object is met, wherein the second predetermined condition comprises that the deterrent was output based on the object being located in a predetermined area within a field of view of the active reflected wave detector, and the object remains in said predetermined area after a predetermined time period has elapsed after output of said deterrent; and if the second predetermined condition is met, control the output device to output a second deterrent, wherein the second predetermined condition is based at least on a location of said object in the environment. process further measured wave reflection data accrued by the active reflected wave detector to determine whether, after a predetermined time period has elapsed after output of the deterrent, a second predetermined condition in respect of the object is met, wherein the second predetermined condition comprises that the deterrent was output based on the object being located in a predetermined area within a field of view of the active reflected wave detector, and the object remains in said predetermined area after a predetermined time period has elapsed after output of said deterrent; and if the second predetermined condition is met, control the output device to output a second deterrent, wherein the second predetermined condition is based at least on a location of said object in the environment.