There are other ways, Prepositioning equipment for the Serbs to be trained on the latest weaponry with Russian Advisors etc... They don’t have to send in units, only advisors with cargo, standard trade.
The pre-positioning of equipment is going to be noticed. You forget that Serbia is still under treaty compliance with the Vienna Document. Large consignments of weaponry isn’t going to go unnoticed.
How are they going to get the equipment into Serbia. Flight plans would have to be filed with the details of the cargo and especially with ordnance etc. A bit of a giveaway when large number of transport flight try to cross NATO airspace. NATO simply puts a stop to it by closing the airspace. How are they going to get it across land borders without inspection? It is back to the airspace problem. They require permission and flight plans to go through the airspace are are subsequently hamstrung!