The other option is B&H, which has a sizeable Serbian population (complete with its own security forces) and who have been causing problems (but what else is new for Serbia.) Kosovo does seem the more likely target, but it has a small NATO force (around 2600 strong) acting as peacekeepers.
The most likely option of what is going on is that it is a desperate move to try and distract NATO away from Ukraine at the request of Putin. Except that it isn't going to work. Serbia isn't going to start anything and Russia can't really support them.
Less likely option is that Serbia may start something and get curbstomped by NATO. Russia will rant and rave, saying they need to mobilise to protect Serbia, but then throw Serbia under the bus and use the mobilisation in an effort to save their floundering Ukrainian invasion and go nowhere near NATO.
KFOR have come out and said that they are prepared to intervene if the Serbs start anything.