The inconspicuous devices are sometimes installed at fixed points, as the DEA has been doing in several border states, but they’re most often mounted on local police cruisers, where they automatically scan and record every license plate that comes within range of their optical sensor.There's discussion at the link as to whether or to what extent this is an invasion of privacy. For the cell phone, see Cory Doctorow's post at BoingBoing:
When they pass an LPR-equipped police car, drivers both innocent and guilty have their whereabouts recorded and tagged with GPS coordinates, along with a color photo and a time stamp. The resulting information is often kept for years, allowing law enforcement to engage in a kind of retroactive surveillance to find out who was where, and at what time.
The data is collected and accessed without the need for warrants or probable cause, because courts have so far held that a license plate – which, after all, is posted very clearly on every driver’s bumper – can’t be considered private information. Privacy advocates think the courts may reevaluate that stance, as LPR systems become so widespread that they allow for tracking on a massive scale.
Cell phones are tracking devices that make phone calls. It’s sad, but it’s true. Which means software solutions don’t always matter. You can have a secure set of tools on your phone, but it doesn’t change the fact that your phone tracks everywhere you go. And the police can potentially push updates onto your phone that backdoor it and allow it to be turned into a microphone remotely, and do other stuff like that. The police can identify everybody at a protest by bringing in a device called an IMSI catcher. It’s a fake cell phone tower that can be built for 1500 bucks. And once nearby, everybody’s cell phones will automatically jump onto the tower, and if the phone’s unique identifier is exposed, all the police have to do is go to the phone company and ask for their information...More info at the link, and many useful reader comments.
...remember that whatever governments can do with technology, organized criminals can do too (this is doubly true of back-doors that governments mandate in telecoms equipment and software to make spying easier -- they can be used by anyone, not just "good guys").