Indeed there is 2% Germanic R1b in Serbia. I assumed according to the R1b-S21 map that the northern R1b subclades were totally absent (in favor to the South Eastern R1b). If you see Maciamo, well, recommend an update on R-S21.
It is true that there is very little data about R1b-S21 in the Balkans at the moment. In all logic R1b-S21 should be found within the 1-5% range in many places in Southeast Europe considering that I1 is found everywhere and I2b1 in many locations too.