Various implementations of an occupant detection system may be used in a vehicle to detect the presence of a living occupant (human or otherwise) and generate a warning. The warning may be communicated to another person(s) or to other vehicle systems to alert people in the vicinity of the vehicle. The system prevents injury and death to people and pets that may be accidentally within a parked car and unable to egress. The system may be integrated into a new vehicle or housed in a separate device that can be plugged into a power outlet within the vehicle.