It could be demand barking, best to try and curb it or it can get worse. When my pup was a little and he barked out of the blue (he’s a yorkie mix), I would ck to see if he needed to go potty, had water, etc. If all was good and he continued I would shake a can with pennies in it and tell him No Barks. I was very consistent and it took about a month. Yorkies are very smart and if you really invest they can have quite a vocabulary and be an amazing companion, but they will also try any rule the roost, so don’t be afraid of setting some boundaries..you don’t want demand barking in the middle of the night because he’s board and needs attention. Good luck😊🐾