Secret treaties were absolutely commonplace for much of recorded history – hence why it was such a big deal when Woodrow Wilson put banning them as one of the first of his Fourteen Points during WWI . According to Chad Khal, there were no less than 593 such treaties made between 1521 and 2004.
As for witnesses, that’s a bit more unusual. In no small part because they were so common, states just treated them like normal treaties, requiring only the signatures of the parties involved. You’d hardly want a lot of witnesses to a secret treaty, lest you compromise the treaty’s secrecy.