The various implementations disclosed herein pertain to methods and systems for detecting a state of monitored objects in a premises. In one aspect, a method is performed at a computer system that is communicably coupled to sensor devices. The computer system receives and processes information of acoustic signals against a database of stored acoustic signatures characterizing predefined acoustic signals generated by electrically non-powered passive tags in response to physical motion of respective monitored objects associated with the passive tags. It receives a first acoustic signal characterized by a first acoustic signature and generated by a first passive tag. In response to the receiving, based on the first acoustic signature and information in the database, it identifies the first monitored object associated with the first acoustic signature, determines a first state of the first monitored object, and provides a notification of the first state to an occupant of the premises.