They are now asking for the internet tech community for help in order to identify the device. Below verbatim is the plea for help published on torproject website. The fine article also contains pictures of the device.
"On March 4th, 2015, we found a tracking device inside of the wheel well of a car belonging to an attendee of the Circumvention Tech Festival in Valencia, Spain. This was reported in the local media.
If you have information about this device — please send information to jacob at appelbaum dot net using gpg.
The device was magnetically mounted inside of the left wheel well of the car. The battery is attached by cable to the tracking device. The battery was magnetically mounted to the frame of the car. The tracking device was similarly magnetically mounted. The device itself has an external magnetically mounted GPS antenna. It has a very simple free hanging GSM antenna. The device included a Movistar SIM card for GSM network access. The entire device was wrapped in black tape.""
Link to Original Source