I would think they'd get warrants to go through every piece of information about that man - like the bank records for the account they deposited Castle's money into to hire him.

If they could get bank account and routing numbers for everyone who ever hired him, I would think ONE of those would belong to someone with a name Beckett or her father would recognize......and I see no reason why it should take until next season! If she wants to find out badly enough, she should pursue it. They could start investigating in the next episode!