1. A vehicle-mountable thermal imaging system comprising: an infrared imaging device configured to be installed in an interior compartment of a vehicle and comprising a focal plane array (FPA) configured to capture thermal images comprising pixels representing thermal radiation variations in at least a portion of the interior compartment; and a processor configured to: analyze the thermal radiation variations represented in the pixels of the thermal images; detect one or more regions-of-interest (ROIs) in the thermal images based on the analysis of the thermal radiation variations; analyze geometric and thermal properties of the one or more detected ROIs; determine, based on the analysis of the geometric and thermal properties of the detected ROIs, temperature profiles of one or more objects present in the portion of the interior compartment and approximate sizes and/or shapes of the one or more objects; and determine whether one or more unwanted occupants are present in the interior compartment based on the determination of the temperature profiles and the approximate sizes and/or shapes of the one or more objects.