## 1 Introduction
### Motivation  
Vehicle idling refers to when a vehicle's engine is running at a minimal steady state rotations per minute (rpm) and no external torque demand (e.g., while the vehicle is stationary). It happens for various reasons, which can be unintentional or intentional. Some common benign reasons for vehicle idling include traffic stops, engine warmup in cold weather, loading/unloading passengers or cargo, use of vehicle accessories, stop-and-go situations, mechanical issues, and safety situations [1; 2]. Idling for long periods is generally discouraged due to the environmental and financial impact (wasting fuel and emitting pollutants) as well as potential wear on engine components. However, idling vehicles can also potentially pose a security or terrorist risk, although this is relatively rare and typically depends on the specific context, location, and the intentions of the individuals involved. There are several ways in which idling vehicles could be exploited for malicious purposes, such as delivery of explosives [3], creation of a security breach, covert surveillance or reconnaissance of restricted areas, thwarting access by creating roadblocks, disruption of traffic, triggering devices for igniting their fuel tanks, and ramming attacks [4]. An idling vehicle might be used for a quick getaway after committing a crime or carrying out surveillance. For example, criminals might leave a vehicle running in a location and use it to flee the scene of a robbery, theft, or a violent attack [5]. The fact that the vehicle is already idling means that it is ready to go at a moment's notice, which is a potential risk in areas of high crime or terrorism concerns. In some cases, an idling vehicle could be used as part of a distraction or diversionary tactic. Criminals or terrorists may leave the vehicle running in a specific area to draw attention away from another location where they plan to carry out an attack, such as a bombing or hostage situation. The presence of an idling vehicle could mislead law enforcement or security personnel into focusing on the wrong threat while a different security breach takes place elsewhere. In high-security environments like airports, government buildings, or military installations, an idling vehicle can potentially interfere with security measures such as license plate readers, surveillance cameras, or sensors. The vehicle may be used to block observance of other suspicious activities or to act as a shield for illicit activities happening nearby. An idling vehicle can also indicate pre-crime planning. If the vehicle is parked near the location of a planned crime and running, it suggests that the criminals are preparing to engage in the crime. After crime execution, the idling vehicle can be used to escape quickly, reducing the chance of being caught in the act. If the crime is part of a larger operation (e.g., organized crime, gang activities), multiple idling vehicles could be involved, with some acting as lookouts while others are used for the actual getaway. The idling vehicle serves as a point of escape for the criminal group or gang, ensuring that the criminals have access to a fast exit once they have completed their criminal activity. If a vehicle has been idling for a while and then suddenly changes operating state, it could signal that the occupants inside the vehicle are preparing to leave or move--a potentially suspicious act. In a security-sensitive area, this could be interpreted as a change in intent, raising alarms. In high-security areas, vehicles often need to be pre-approved to enter or to park. If a vehicle starts up unexpectedly without visible signage or clear permission, it could suggest that the vehicle or driver is attempting to evade security measures or is unauthorized to be in the area. More persons inside a vehicle can increase the threats in several ways, especially when considering factors such as security risks, traffic safety, public perception, and potential for criminal or terrorist activities.