The GLK has officially been replaced by the GLC, but its final transition is yet to be completed.
We saw the GLC coupe at the Shanghai Auto Show earlier in the year, since then we've come to know that Mercedes will debut the new coupe in 2016 as a 2017 model.
The truck spotted in question is believed to be the new GLC AMG Sport, which falls short of full AMG but is more then a 'regular' GLC. A strategy similar to BMW's X4 M40i.
Expect the GLC450 to offer the same 3.0L twin-turbo V6 as other AMG Sport models.