The invention claimed is:1. A control device of a luminaire comprisinga controller for controlling operation of the luminaire, the controller preprogrammed to make the luminaire assume a certain target lighting level as a response to a triggering local input signal,a communications module for wireless communication with other devices, the communications module configured to receive indication messages from the other devices, andan adaptation module for adjusting the operation of the controller in accordance with received indication messages;wherein the adaptation module is configured todetermine a relationship strength, pertinent to a particular other device and indicative of regularity at which triggering local input signals have been observed after first receiving an indication message from said other device, andreprogram the controller to make the luminaire assume a preparatory lighting level as a response to receiving a further indication message from said other device, the preparatory lighting level being dependent on the determined relationship strength.2. A control device according to claim 1, wherein the adaptation module is configured to:classify a number of previously received indication messages from said other device into time windows according to the time passed between receiving the indication message and subsequently observing a triggering local input signal, anduse the relative numbers of previously received indication messages in said time windows as a basis for determining said relationship strength.3. A control device according to claim 2, wherein the adaptation module is configured to:convert the relative number of previously received indication messages in each time window into a set of descriptor values, wherein each of said descriptor values is a quantitative descriptor of how well the relative number of previously received indication messages matches a particular quantity class, so that said set has as many descriptor values as there are quantity classes, anduse the sets of descriptor values as a basis for determining said relationship strength.4. A control device according to claim 3, wherein the adaptation module is configured to:apply a set of selection rules to pick, from the sets of descriptor values, a specific value for each of a number of output membership functions, anduse the specific values of said output membership functions as a basis for determining said relationship strength.5. A control device according to claim 4, wherein the adaptation module is configured to:calculate a weighted average of said specific values of said output membership functions, wherein weights used for said weighted average represent at least the relative magnitudes of said specific values, anduse the calculated weighted average as the value of said relationship strength.6. A control device according to claim 1, wherein the controller is configured to:respond to the reception of a further indication message from said other device by making the luminaire assume a preparatory lighting level that lies between a passive lighting level and said target lighting level, the relative difference between said preparatory lighting level and said passive lighting level in relation to the difference between the target lighting level and said passive lighting level being proportional to said relationship strength.7. A control device according to claim 1, whereinthe communications module is configured to transmit indication messages, andthe controller is configured to make the communications module transmit an indication message in response to said triggering local input signal.8. A luminaire comprising one or more light sources and a control device, the control device comprisinga controller for controlling operation of the luminaire, the controller preprogrammed to make the luminaire assume a certain target lighting level as a response to a triggering local input signal,a communications module for wireless communication with other devices, the communications module configured to receive indication messages from the other devices, andan adaptation module for adjusting the operation of the controller in accordance with received indication messages;wherein the adaptation module is configured todetermine a relationship strength, pertinent to a particular other device and indicative of regularity at which triggering local input signals have been observed after first receiving an indication message from said other device, andreprogram the controller to make the luminaire assume a preparatory lighting level as a response to receiving a further indication message from said other device, the preparatory lighting level being dependent on the determined relationship strength.9. A method for operating a control device of a luminaire, comprisingdetermining a relationship strength, pertinent to a particular other device and indicative of historical regularity at which triggering local input signals have been observed after first receiving an indication message from said other device, andreprogramming a controller contained in said control device to make the luminaire assume a preparatory lighting level as a response to receiving a further indication message from said other device, the preparatory lighting level being dependent on the determined relationship strength.10. A method according to claim 9, comprising:classifying a number of previously received indication messages from said other device into time windows according to the time passed between receiving the indication message and subsequently observing a triggering local input signal, andusing the relative numbers of previously received indication messages in said time windows as a basis for determining said relationship strength.11. A method according to claim 10, comprising:converting the relative number of previously received indication messages in each time window into a set of descriptor values, wherein each of said descriptor values is a quantitative descriptor of how well the relative number of previously received indication messages matches a particular quantity class, so that said set has as many descriptor values as there are quantity classes, andusing the sets of descriptor values as a basis for determining said relationship strength.12. A method according to claim 11, comprising:applying a set of selection rules to pick, from the sets of descriptor values, a specific value for each of a number of output membership functions, andusing the specific values of said output membership functions as a basis for determining said relationship strength.13. A method according to claim 12, comprising:calculating a weighted average of said specific values of said output membership functions, wherein weights used for said weighted average represent the relative magnitudes of said specific values, andusing the calculated weighted average as the value of said relationship strength.14. A non-transitory memory storing a computer program for operating a control device of a luminaire, the computer program including one or more sequences of one or more instructions which, when executed by one or more processors, cause the control device to at least perform the following:determining a relationship strength, pertinent to a particular other device and indicative of historical regularity at which triggering local input signals have been observed after first receiving an indication message from said other device, andreprogramming a controller contained in said control device to make the luminaire assume a preparatory lighting level as a response to receiving a further indication message from said other device, the preparatory lighting level being dependent on the determined relationship strength.