Maybe you stumbled upon Bulgaria's "Area 51"
That's not as daft as it seems. There are a lot of military research establishments and other government installations dotted around Bulgaria dating from the Cold War. These are now unused and slowly going derelict, but they still have their paid security guards. (This provides useful employment in some areas.) These places are quite often hidden in woods. It's possible that you wandered near to one of these and gave a bored guard something to do.
Fruit plantations, vinyards and the like also have their guards when the fruit is in season.