In some cases the person who recommended you may be bound by an agreement not to try and hire any previous colleagues for a period of time (I had to sign one wh

In some cases the person who recommended you may be bound by an agreement not to try and hire any previous colleagues for a period of time (I had to sign one when I quit Google), in which case it would probably violate the agreement for you to know that said person was the one who recommend you. While I was at Google they had at least one event in which people were encouraged to think of anyone that would possibly be worth contacting, so it's possible that your name came in through that avenue (and the person who recommended you had signed the aforementioned agreement).