When police cannot get the data they need from mobile phone carriers, they start implementing some other measures in order to be able to know the locations of mobile phones or even listen to the phone calls. Your case is the one that demonstrates one of their methods.
Stingray is a cell phone tracking device developed by Harris Wireless Products Group. It costs a lot of money but its effectiveness is very high. The Stingray system is very clever and cunning mobile phone tracking equipment and I guess you understand that it is used with some reason, probably funded by governmental organizations like NSA or other.
The working principle of Stingray is quite simple and brilliant at the same time. This system pretends that it is a cell phone tower, making mobile phones around believe it and connect to it without any suspicions. After that Stingray can track the location of any cell phone connected to it and even record the numbers which were dialed from those cell phones. Still it cannot record the content of the call itself, so you won't be wiretapped.
As long as this tracking device is using mobile phone frequencies for its work, you can prevent Stingray from tracking your cell phone and keep your location safe and private.