Can anyone shed any light as to what this is (circled in white)?
This is a military test transponder, not a real aircraft. The callsign GAFTT08 stands for German Air Force Test Transponder 08. When you zoom in on the map, you can see the reconstructed position move in a pattern around the fixed true position:
As far as I can tell, the military does not publicly say what exactly they are doing here. The only sources I could find are from forums:
GAFTTxx = German Air Force Test Transponder
German Air Force Test Transponder. So not an aircraft, not a drone. Don’t know why it is online all the time, but it is. Sometimes over Würzburg, sometimes over Ingolstadt.
Additionally, the squawk code of 7777 is used in Germany for fixed test transponders according to Wikipedia (thanks to ymb1 for pointing this out):
7777 US, Germany, Belgium, Netherlands: Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE)
This is also confirmed by the following ICAO definition:
Code 7700 is reserved for recognizing an aircraft in emergency. (Codes 7711 to 7717 and 7721 to 7727 are reserved for SAR operations and code 7777 for monitoring the ground transponder.)
(ICAO - SSR CODE ASSIGNMENT SYSTEM, emphasis mine)
It is a test transponder for the German Air force. According to Wikipedia, it is a test for the friend or foe identification system. So, basically they are testing technology to identify aircraft, so they can aviod friendly fire in battle situations. It is not a real aircraft.