What a mysterious and intriguing dog!
Your Border Collie guess might be right. Long ago our Border Collie-Lab mix pup was also silent but for one single bark. Stayed silent for a little more than two and a half years. Learned hand and voice commands very early and quickly. Awesome smart dog.
Then he started barking, mostly alerts for his stupid humans who didn’t respond to his action signals. Also responded to sirens with his own siren, awesome range, deep deep bass and climbed. He had a darn near perfect doppler effect. Always amazed guests with that. Then he started ‘talking’ looking us in the eye followed with a series of grumbles, mumbles and ending with a deep inhale and exhale that flapped his flews. End of discussion.
Also there was very strong evidence of his amazing memory. Always scanned the whole room as he entered it. If anything had been moved or there was something new, even small things, he investigated it first thing. Did they same thing with visits to friends homes.
Miss that dog.