When so is said at the end of a thought, it seems to say "So that is my explanation; that is my reason."
The word so can mean "for that reason" or "therefore" when it is used as a conjunction. In this case, at the end of a sentence that does not go on, so means "therefore."
And when so is the last thing said, we view it as trailing off. Since it means "therefore," there is a semicolon or period in front of it. It can be followed by a dash, indicating the sentence did not get finished, or ellipses
...I had contacted her and given her the data; so --
...I had contacted her and given her the data; so...
Happy punctuating!
Margie