If this was done as part of a criminal scheme then I would presume that the scheme would make it illegal. However it seems that the motivation for these actions is unknown. Is there any law that would make placing an airtag on a car or coat of an individual without their knowledge or consent illegal? Any jurisdiction would be of interest.
It's clearly allowable with a warrant
It's OK for police to track someone if they have a warrant. But for the owner, it is also not illegal to throw the device away, as two cases involving Law Enforcement show:
Some two years or so ago, Indiana police had a warrant to mount a tracking device on a car. The owner found it and removed it. Police got a warrant and sued the owner for theft. But the Indiana Supreme Court decided: No, that's not theft, and atop that, the warrants for searching for that device are invalid.
In a similar case, a woman in Lousiana did find a tracker. Aparently, she watched the device being planted, and when she reported that to the police, they requested the device - which would have got the woman in trouble for being in possession of a device she's not allowed to have, as tracking cars without consent is illegal in Lousiana. Somehow, it ended on a pole across from a school - and they have pretty much nothing to sue for in that case.
So, is it legal for laymen?
Which brings us to the actual question at hand: Is it illegal to track other cars? It could be even illegal to possess an airtag in lousiana under LA Rev Stat § 14:222.3 (2017), unless you have a certain exception, like are a police unit investigating or to track your car (see subsection C):
(1) It shall be unlawful for any person to possess a cellular tracking device or to use a cellular tracking device for the purpose of collecting, intercepting, accessing, transferring, or forwarding the data transmitted or received by the communications device, or stored on the communications device of another without the consent of a party to the communication and by intentionally deceptive means.
Note that Bluetooth is still in the definition of Communications Device in that law, and the phone reading the location data from the apple Airtag together form a pair of CTD and TD:
(1) "Cellular tracking device" means a device that transmits or receives radio waves to or from a communications device in a manner that interferes with the normal functioning of the communications device or communications network and that can be used to intercept, collect, access, transfer, or forward the data transmitted or received by the communications device, or stored on the communications device; includes an international mobile subscriber identity (IMSI) catcher or other cell phone or telephone surveillance or eavesdropping device that mimics a cellular base station and transmits radio waves that cause cell phones or other communications devices in the area to transmit or receive radio waves, electronic data, location data, information used to calculate location, identifying information, communications content, or metadata, or otherwise obtains this information through passive means, such as through the use of a digital analyzer or other passive interception device; and does not include any device used or installed by an electric utility solely to the extent such device is used by that utility to measure electrical usage, to provide services to customers, or to operate the electric grid.
(2) "Telecommunications device" means any type of instrument, device, or machine that is capable of transmitting or receiving telephonic, electronic, radio, text, or data communications, including but not limited to a cellular telephone, a text-messaging device, a personal digital assistant, a computer, or any other similar wireless device that is designed to engage in a call or communicate text or data. It does not include citizens band radios, citizens band radio hybrids, commercial two-way radio communication devices, or electronic communication devices with a push-to-talk function.
So to track someone else's car? Only if you have the permission of the tracked person under C4, C5, or C12, or are exempt under C6 or C10, or you are the parent of a minor under C7 (which would often include the C4/C5 but overrides consent).
This is just about to become a hot topic in policing; for example, a bill is being proposed to prohibit such invasions of privacy:
“As a longtime proponent of public safety and privacy measures, state Rep. John Galloway of [Pennsylvania] is introducing legislation that would seek to prohibit Apple AirTags from being used outside of their intended use as a locator for misplaced personal items.”
In other states, a similar loophole may exist.