A method of detecting a state of monitored objects (1002, 1004, 1008), comprising: at a computer system having one or more processors and memory storing instructions for execution by the one or more processors, wherein the computer system is communicably coupled to one or more sensor devices: obtaining a database (836, 1100, 1200) of stored acoustic signatures (1012, 1014) characterizing predefined acoustic signals, the predefined acoustic signals generated by electrically non-powered passive tags (1010, 1016) in response to physical motion of respective monitored objects (1002, 1004, 1008) associated with the passive tags (1010, 1016); receiving a first acoustic signal characterized by a respective acoustic signature (1012, 1014) and generated by a first electrically non-powered passive tag of the passive tags (1010, 1016) in response to physical motion of a first monitored object of the respective monitored objects (1002, 1004, 1008), wherein the first monitored object is associated with the first passive tag (1010, 1016); and in response to the receiving: based on the respective acoustic signature (1012, 1014) and information in the database (836, 1100, 1200): identifying the first monitored object (1002, 1004, 1008) associated with the respective acoustic signature (1012, 1014); and determining a first state of the first monitored object (1002, 1004, 1008); storing in the database (836, 1100, 1200) the determined first state of the first monitored object (1002, 1004, 1008); and providing an indication of the first state of the first monitored object (1002, 1004, 1008) to a monitoring service associated with the one or more monitored objects (1002, 1004, 1008). at a computer system having one or more processors and memory storing instructions for execution by the one or more processors, wherein the computer system is communicably coupled to one or more sensor devices: obtaining a database (836, 1100, 1200) of stored acoustic signatures (1012, 1014) characterizing predefined acoustic signals, the predefined acoustic signals generated by electrically non-powered passive tags (1010, 1016) in response to physical motion of respective monitored objects (1002, 1004, 1008) associated with the passive tags (1010, 1016); receiving a first acoustic signal characterized by a respective acoustic signature (1012, 1014) and generated by a first electrically non-powered passive tag of the passive tags (1010, 1016) in response to physical motion of a first monitored object of the respective monitored objects (1002, 1004, 1008), wherein the first monitored object is associated with the first passive tag (1010, 1016); and in response to the receiving: based on the respective acoustic signature (1012, 1014) and information in the database (836, 1100, 1200): identifying the first monitored object (1002, 1004, 1008) associated with the respective acoustic signature (1012, 1014); and determining a first state of the first monitored object (1002, 1004, 1008); storing in the database (836, 1100, 1200) the determined first state of the first monitored object (1002, 1004, 1008); and providing an indication of the first state of the first monitored object (1002, 1004, 1008) to a monitoring service associated with the one or more monitored objects (1002, 1004, 1008). obtaining a database (836, 1100, 1200) of stored acoustic signatures (1012, 1014) characterizing predefined acoustic signals, the predefined acoustic signals generated by electrically non-powered passive tags (1010, 1016) in response to physical motion of respective monitored objects (1002, 1004, 1008) associated with the passive tags (1010, 1016); receiving a first acoustic signal characterized by a respective acoustic signature (1012, 1014) and generated by a first electrically non-powered passive tag of the passive tags (1010, 1016) in response to physical motion of a first monitored object of the respective monitored objects (1002, 1004, 1008), wherein the first monitored object is associated with the first passive tag (1010, 1016); and in response to the receiving: based on the respective acoustic signature (1012, 1014) and information in the database (836, 1100, 1200): identifying the first monitored object (1002, 1004, 1008) associated with the respective acoustic signature (1012, 1014); and determining a first state of the first monitored object (1002, 1004, 1008); storing in the database (836, 1100, 1200) the determined first state of the first monitored object (1002, 1004, 1008); and providing an indication of the first state of the first monitored object (1002, 1004, 1008) to a monitoring service associated with the one or more monitored objects (1002, 1004, 1008). based on the respective acoustic signature (1012, 1014) and information in the database (836, 1100, 1200): identifying the first monitored object (1002, 1004, 1008) associated with the respective acoustic signature (1012, 1014); and determining a first state of the first monitored object (1002, 1004, 1008); identifying the first monitored object (1002, 1004, 1008) associated with the respective acoustic signature (1012, 1014); and determining a first state of the first monitored object (1002, 1004, 1008); storing in the database (836, 1100, 1200) the determined first state of the first monitored object (1002, 1004, 1008); and providing an indication of the first state of the first monitored object (1002, 1004, 1008) to a monitoring service associated with the one or more monitored objects (1002, 1004, 1008).