A system for monitoring a state of occupancy of a pre-determined area by at least one live presence, the system comprising: a first sensor (SENS1) comprising a first time counter configured to count at a given counting frequency, the first sensor being configured to detect at least one movement of a live presence within a first pre-determined zone within the pre-determined area; at least one second sensor (SENS2) comprising a second time counter configured to count at substantially a same counting frequency as the counting frequency of the first counter, the second sensor being configured to detect at least one movement of a live presence within a second pre-determined zone within the pre-determined area; and a server having access to a reference time counter (TREF) and to topographic information concerning at least the first and second pre-determined zones, said reference time counter (TREF) being configured to count at substantially a same counting frequency as the counting frequency of the first counter; wherein the first sensor and the second sensor are battery-operated passive infra-red motion sensors; wherein the resolution of the information provided by the first sensor and the second sensor is equal to 1 pixel; wherein the first sensor is further configured to maintain a first log comprising, for each movement detected by the first sensor, a corresponding first event timestamp having a value which is based on a value of the first time counter when said movement within the first pre-determined zone is detected; wherein the second sensor is further configured to maintain a second log comprising, for each movement detected by the second sensor, a corresponding second event timestamp having a value which is based on a value of the second time counter when said movement within the second pre-determined zone is detected; wherein the server is further configured to receive the first and second logs via a wireless communications network, the first and second sensors being further configured to attach or otherwise insert, respectively, a first and a second send timestamp to their corresponding logs, the first and second send timestamps having values derived from their respective time counters when the respective logs are sent; wherein the server is further configured to analyse the first and second logs to provide the state of occupancy of the pre-determined area, said analysis by the server including combining the topographic information of the first and second pre-determined zones, the first and second send timestamps synchronised according to the reference time counter (TREF) and the event timestamps comprised in the first and second log of the first and second sensors. a first sensor (SENS1) comprising a first time counter configured to count at a given counting frequency, the first sensor being configured to detect at least one movement of a live presence within a first pre-determined zone within the pre-determined area; at least one second sensor (SENS2) comprising a second time counter configured to count at substantially a same counting frequency as the counting frequency of the first counter, the second sensor being configured to detect at least one movement of a live presence within a second pre-determined zone within the pre-determined area; and a server having access to a reference time counter (TREF) and to topographic information concerning at least the first and second pre-determined zones, said reference time counter (TREF) being configured to count at substantially a same counting frequency as the counting frequency of the first counter; wherein the first sensor and the second sensor are battery-operated passive infra-red motion sensors; wherein the resolution of the information provided by the first sensor and the second sensor is equal to 1 pixel; wherein the first sensor is further configured to maintain a first log comprising, for each movement detected by the first sensor, a corresponding first event timestamp having a value which is based on a value of the first time counter when said movement within the first pre-determined zone is detected; wherein the second sensor is further configured to maintain a second log comprising, for each movement detected by the second sensor, a corresponding second event timestamp having a value which is based on a value of the second time counter when said movement within the second pre-determined zone is detected; wherein the server is further configured to receive the first and second logs via a wireless communications network, the first and second sensors being further configured to attach or otherwise insert, respectively, a first and a second send timestamp to their corresponding logs, the first and second send timestamps having values derived from their respective time counters when the respective logs are sent; wherein the server is further configured to analyse the first and second logs to provide the state of occupancy of the pre-determined area, said analysis by the server including combining the topographic information of the first and second pre-determined zones, the first and second send timestamps synchronised according to the reference time counter (TREF) and the event timestamps comprised in the first and second log of the first and second sensors.